Historical Development of the Trinity Doctrine – Fifth Century Arianism

This is an article in the series on the historical development of the Trinity doctrine. The current article considers the Fifth Century but also provides an overview of the events of the preceding centuries.


Pre-Nicene Church Fathers


The church fathers of the first 300 years were not Trinitarians. The Trinity doctrine was only developed in the fourth and fifth centuries. For the pre-Nicene church fathers, in general, the Father was “the only true god.” They had an extremely high view of Christ, for example, that He was “born of the very substance of the Father” “before time began” but they nevertheless regarded the Son as subordinate to the Father, who is “the Head of Christ.” For example, see Irenaeus.

Nicene Creed


After Emperor Constantine legalized Christianity in the year 313, he has an important role in the formulation of the Nicene Creed of 325 in that he allowed a small minority to determine the final form of that creed. That creed elevates the Son to “true god from true god.”

(I use the word “god” because the ancient languages did not distinguish between upper- and lower-case letters. Consequently, the ancient languages did not have a word equivalent to the modern word “God.” In Greek, they used the word theos, which is a general word used for all gods.) 

Fifty year Arian period

However, after the Council of Nicaea in the year 325, the Arian controversy continued. That council rejected Arius’ views, but it caused a second controversy; not between Arius and the rest, but between the followers of Origen and the minority party that added the word homoousios with Constantine’s blessing. (See – Eusebius of Caesarea)

Soon after 325, Emperor Constantine became convinced that the Nicene Creed is not Biblical. During the next 50 years, all Christian emperors rejected the Nicene Creed. Consequently, the church also rejected the Nicene Creed. (Religious freedom was not part of Roman culture.) During the next 50 years, there was a range of alternative theories about the relationship between God and His Son. Trinitarians often cluster them all under the title Arianism but most of them were very different from what Arius taught.

Several alternative creeds were accepted by the church during those 50 years. While the Nicene Creed sought to emphasize equality between the Father and Son, in all of the creeds formulated in the 50 years after Nicaea, the Son was regarded as subordinate to the Father. See, for example, the Long Lines Creed.

During that fifty-year period, the church converted many Germanic peoples to Christianity and taught them a non-Nicene Christology. Also at this time, Germanic people began to migrate into the Roman Empire in large numbers.

However, in the year 380, Theodosius became emperor. Being a zealous supporter of Nicene Christianity, he immediately outlawed all other forms of Christianity. He persecution nwas so effectively that non-Nicene Christology disappeared among the elite in the empire. However, the Germanic peoples (called ‘barbarians’ by the Romans) remained Arian.

Barbarian domination of the West

After Theodosius died in 395, the Germanic immigrants, due to their numbers and military prowess, became a dominant force in the Roman Empire. They tolerated figurehead Western Roman Emperors until 476. However, in that year a Germanic chieftain deposed the last Western Roman Emperor. They then divided the territory of the western provinces between the Germanic tribes. However, these tribes continued to function as part of the Roman Empire. Since these Germanic tribes were non-Nicene, the Western Roman Empire was once again non-Nicenedominated.

The Roman Church in the Fifth Century

The Roman Church survived throughout this turmoil. One reason is that the Germanic tribes wished to remain part of the Roman Empire and tolerated the Roman Church because it was an official part of the Roman system of government. For that reason also, the Roman Church actually grew in strength.

One consequence of the growing strength of the Church in Rome was that the Germanic peoples converted to the Nicene Church, rather than to Arianism. The Franks were the first to convert. The other nations converted to Nicene Christianity over the subsequent centuries.


The purpose of this article series is to show that the religious preferences of the Roman Emperors determined the Christology of the church. The fact that the church today is dominated by the Trinity doctrine is the direct result of decisions taken by Roman Emperors. The current article refers to the roles which Constantine and Theodosius played. The next article discusses Justinian’s wars on the Arians.


Church Fathers – First 300 Years

The Church Fathers of the first 300 years were not Trinitarians. For them, the Father alone was the “Lord God Almighty,” “the only true god, the unbegotten and unapproachable” and the “Lord of the universe.”

But they also had an extremely high view of Christ: They wrote that He was “born of the very substance of the Father” “before time began.” “Every knee should bow” before Christ Jesus. But that is not because Jesus is the Almighty, but because it is “the will of the invisible Father.” In other words, in their view, the Son is subordinate to the Father, who is the only true ‘god’. Justin Martyr explicitly put Jesus “in the second place” next to God. Irenaeus, quoting the New Testament, refers to the Father as “the Head of Christ.” Polycarp, also quoting the Bible, identified the Father as Jesus’ God (e.g. Rev. 3:12).

Nicene Creed (325)

After Christianity was legalized in 313, emperor Constantine played a huge role in the formulation of the Nicene Creed of 325. While the Bible and the early fathers described the Father as “the only true god,” the Nicene Creed elevated the Son as Homoousios (same substance) with the Father and as “true god from true god.” This elevates the Son to near equality with the Father. The article Nicene Creed discusses whether that creed declares the Son to be fully EQUAL to the Father.

The words “God” and “god”

The reader might be surprised by the references in this article to “god” rather than to “God.” The reason is that ancient languages did not distinguish between upper- and lower-case letters. Consequently, the Bible writers and these early fathers did not have a word that is exactly equal to the modern word “God,” which we use today as a name for one specific Being; the uncaused Cause of all things. The ancient word which they used (theos in Greek) had a more general meaning and is equivalent to the modern word “god.” They used that same word for the Greek gods.

These early writers (Ignatius, Irenaeus, etc.), therefore, literally referred to the Father as “the only true god,” but to Jesus as “our god. To translate theos as “God,” with a capital “G,” is an interpretation. Translators today, generally, assume the Trinity doctrine in which Jesus Christ is equal with the Father. Both are regarded as the uncaused Cause of all things. Such translators translate theos, when it describes Jesus, also as “God.” 

It is important to know that that is an application of the Trinity doctrine and does not necessarily reflect the intention of the early writers.

As shown above, the earliest church fathers had an extremely high view of Christ but did not regard Christ as equal to the Father. For that reason, it may be preferable to use the word “god” rather than “God.” That might reflect the intentions of these ancient writers better. For example, the phrase “true God” is a tautology, for there is only one true God. But “true god” is a logical phrase. See the article Jesus is our God for a further discussion of this crucial principle.

Fifty-years Arian Period (330-380)

The Council of Nicaea did not end the Arian controversy. The bishops went on teaching as they had before. Within a few years after Nicaea, Church leaders convinced emperor Constantine that the Nicene Creed was not Biblical. During the 50 years after Nicaea, the emperors were Arian. For that reason, Arianism dominated the church in that period (See Fourth Century Arian Period.). 

Religious freedom was not part of the culture of the Roman Empire. Just like Constantine exiled all church leaders who did not accept the Nicene Creed, the emperors after Constantine viciously persecuted the church leaders who taught the Nicene Creed. 

Many alternative creeds were formulated during that 50-year Arian period, for example, the Long Lines Creed.

During those fifty years, the Gothic convert and Arian bishop Ulfilas went as a missionary to the Gothic tribes across the Danube. Ulfilas translated the Bible in Gothic and had success in converting the Goths to the Arian form of Christianity. The conversion of Goths led to a widespread diffusion of Arian Christianity in the years 340 to 350 among other Germanic peoples as well, such as the Visigoths, the Vandals, the Lombards, Svevi, and Burgundians. (See the Wikipedia page on Arianism and the Britannica pages for Goth and Ulfilas.)

More or less at this time, the Germanic tribes began to migrate in large numbers into the Roman Empire. (See Migration Period.) Rome referred to them as “barbarians,” but they were the people that occupy most of Europe today.

Death of Arianism (380)

In 380, Theodosius became emperor. He was a zealous Nicene Christian and immediately outlawed Arianism. He so effectively persecuted Arianism that it disappeared among the elite in the empire. However, not being subject to the Roman emperor, the Germanic tribes remained Arian.

Barbarian Control of the Western Empire

As discussed in The Fall of Rome, more and more Germanic immigrants were recruited into the Roman army. The Imperial forces became dependent on Germanic soldiers. They were also appointed in top positions in the military of the Western Roman Empire. Since Roman generals always were very influential in the Roman Empire, this put the ‘barbarians’ in a very strong position. 

Theodosius was the last Roman emperor to rule the entire Empire. Soon after his death in 395 Germanic people were, in reality, in charge of the Western Roman Empire. But the Graeco-Roman population still treated them as second-class citizens. Therefore, to obtain equal rights and permanent residency in the empire, the Germanic people revolted against the severe conditions of their tenure in the Roman Empire. They sacked Rome in 410 and again in 455. (See Fall of the Roman Empire.)

Although they dominated the Western Empire already from the beginning of the 400s (the fifth century), they tolerated figurehead Western Roman Emperors until 476. In that year Odoacer—an Arian Germanic chieftain—deposed the last Western Roman Empire. He soon conquered the whole of Italy. During that time, the territory of the Western Empire was divided between the Germanic tribes, particularly the Goths and Vandals. However, to some extent, they still functioned as part of the Roman Empire. In name at least, they were subject to the emperor in Constantinople. For these reasons, historians today prefer to refer to the Transformation of the Western Roman Empire; rather than to its Fall. It was a slow process over decades and even centuries during which the Germanic people wrestled control of the Western Empire from the Romans. 

Theodosius had made an end of Arianism among the Roman people in 380, but now, through the Germanic domination of the Western Roman Empire, it was once again Arian dominated.

The Roman Church in the Fifth Century

The Roman Church could have perished.

The Roman Church survived throughout this period. There are at least two reasons why we might have expected the Church in Rome to perish with the demise of the Western Empire:

Part of the Roman Government 


Firstly, the Church in Rome was part of the government of the Roman Empire.

After emperor Constantine I legalized Christianity in 313 AD, the church became closely married to political powers of the times. It became very different from what we know today as a church: It rather functioned as a government department. The emperor was the real head of the church. He appointed bishops and they were accountable to him. The emperor also had the final say with respect to controversies in the church, such as concerning Christology. For example:

Emperor Constantine had a huge role in the decisions of the Council of Nicaea. He called the council, presided over it, guided the discussions, proposed and enforced the important word Homoousios and exiled all bishops that did not agree.

When Theodosius I became emperor in 380, the imperial capital was solidly Arian. But he immediately outlawed all other forms of Christianity, exiled Arian bishops, and banned Arians from the Council of 381. The 381 Council was simply a formality. (See Death of Arianism.)

Christianity, consequently, became wealthy and the religion of any ambitious civil official.

Secondly, the Church in Rome advocated Nicene Christology, while the Germanic peoples were Arians. 

Since Nicaea – in the year 325 – because the Church functioned as a department of government, these Nicene and Arian Christians often exiled and persecuted one another. Constantine’s successors—the emperors Constantius and Valens actively encouraged the church to reverse the Nicene Creed and they exiled bishops adhering to the Nicene Creed, crushing the Nicene party (see Fourth Century Arian Period). Theodosius, on the other hand, was a Nicene Christian and acted mercilessly against ‘heretics’. He was responsible for the first official executions of Christian ‘heretics’. [Jones 1964, p. 164]


Despite these factors, the new Germanic Arian rulers in the fifth century in the Western Empire allowed the Church in Rome to continue unhindered. Arianism and the Nicene church of the Roman people existed side by side. The Jewish Encyclopaedia describes the situation:

“Most Germanic peoples—such as the eastern and western Goths, as also the Franks, the Lombards, the Suevi, and the Vandals—were baptized into Arian Christianity. These tribes settled in widely spread districts of the old Roman empire. A large number of Jews, already resident in those lands, fell under Arian domination. In contrast with the domination of the orthodox church, the Arian was distinguished by a wise tolerance and a mild treatment of the population of other faiths. This conduct was traceable to some degree to certain points of agreement between the Arian doctrine and Judaism. The very insistence upon the more subordinate relationship of the Son to the God-father is much nearer to the Jewish doctrine of the Messiah than to the conception of the full divinity of the Son, as enunciated at Nicaea.”
(Kohler, Kaufmann; Krauss, Samuel. “ARIANISM”. Jewish Encyclopedia. Kopelman Foundation.)

The Wikipedia – State Church of the Roman Empire states that the tolerance of the Arian Germanic tribes towards other religions resulted in entirely separate Arian and Nicene (catholic) systems of churches and bishops in the previous Western Empire. 

Although the Arian Germanic tribes were generally tolerant towards Nicene Christians, the Vandal regime in North Africa tried to force their Arian beliefs on their North African Nicene subjects, exiling Nicene clergy, dissolving monasteries, and exercising heavy pressure on non-conforming Nicene Christians. This matter will become important when we read of emperor Justinian’s efforts in the sixth century to regain control of the Western Empire, for the first ‘barbarian’ nation which he attacked was the Vandals.


The Arian nations allowed the Roman (Nicene) Church to co-exist unhindered for at least the following reasons:

The Germanic people, after they took control of the Western Empire, intended to remain part of the Roman Empire and the Roman Church was part of the Roman system of government; accountable to the emperor. The Germanic people voluntary—in name at least—subjected themselves to the Roman Emperor, who reigned from the east. 

Religious persecution was part of the Roman culture. Roman emperors always used religion to strengthen the unity of their vast empire and persecuted religions that threaten unity. But, according to Daniel, the Roman Empire “devoured and crushed and trampled” (Dan. 7:7). Perhaps the barbarians were not as barbaric as the Romans.


Actually, instead of perishing, the Church in Rome grew in strength after the ‘barbarians’ wrestled control of the western provinces from the original Graeco-Roman population (Britannica). The reasons include the following:

The Church had a strong, centralized organization: The pope in Rome is the head of the Church. All clergy, including bishops and priests, fell under his authority. Bishops supervised priests; the lowest ranking members of the clergy. For most people, local priests served as the main contact with the Church.

At the same time, after the fall of the Western Roman Empire, there was no single state or government that united all people who lived on the European continent. The transformation of the Western Roman Empire in the fifth century was a time of great political chaos and warfare and the well-organized church became the only stabilizing force. As secular governments came and went, the Papacy remained. The church was a stable force during an era of constant warfare and political turmoil.

The Church also bonded people together. It gave a sense of communal identity. At the local level, the village church was a unifying force in the lives of most people. It served as a religious and social center. Religious holidays, especially Christmas and Easter, were occasions for festive celebrations.


One consequence of the strength and influence of the Church in Rome was that ‘barbarian’ nations converted to the Nicene Church, rather than to Arianism:

The Franks and the Anglo-Saxons also were Germanic peoples but never were Arians. They entered the Western Roman Empire as Pagans.

In 496, Clovis, king of the Franks, converted to Nicene Christianity—as opposed to the Arianism of most other Germanic tribes. Consequently, sometime between 496 and 508, Clovis I forcibly converted the Franks to Christianity. (So much for religious freedom!) This led to widespread conversion among the Frankish peoples across what is now modern-day France, Belgium, and Germany. Three centuries later, it led to Charlemagne‘s alliance with the Bishop of Rome. This was the first of the Germanic peoples to convert to Nicene Christianity.

Æthelberht of Kent did the same for the Anglo-Saxons (see also Christianity in Gaul and Christianisation of Anglo-Saxon England)

Visigothic Spain was Arian until 589. 

The Lombards were Arians until the 7th century.


The first main conclusion from this article is that the religious preferences of the Roman Emperors determined the church’s Christology. Emperor Constantine had a huge role in the formulation of the Nicene Creed. During the next 50 years, the emperors Constantius and Valens enforced Arianism. In 380, Theodosius—a zealous Nicene Christian—became emperor and immediately outlawed Arianism. In the fifth century, Germanic immigrants dominated the Western Empire. Since they were Arians, the West was Arian once again. But, as the next article will show, in the sixth century, emperor Justinian gave Arianism a death wound and it died during the subsequent Byzantine Papacy. The fact that the church today is dominated by the Trinity doctrine is the direct result of decisions taken by Roman Emperors.

The second main purpose of this article is to explain how the Papacy rose to dominance. This topic will be developed further by subsequent articles.


Christology of the persecuted church (First 300 years)
 – Introduction
 – Polycarp
 – Justin Martyr – Current Article
 – Ignatius of Antioch
 – Irenaeus
 – Tertullian – work in progress

 – Origen – work in progress
 – Jesus is our god.
Fourth Century (State Church)
 – Council of Nicaea – A.D. 325 
 – The Nicene Creed Interpreted 
 – Fourth Century Arian Period 

 – What did Arians believe in the fourth century?
 – Long Lines Creed – one of the creeds during the Arian period
 – Death of Arianism – Emperor Theodosius
Fifth Century
 – Fall of the Western Roman Empire
 – Why the Roman Empire fell 
 – The Fall of Rome proves Daniel as a true prophecy.
 – Roman Church grew in strength
Sixth Century
Justinian and the Byzantine Papacy eliminated Arianism.
Middle Ages

 – The massacres of the Waldensians

Emperor Theodosius wiped out Arianism among the Roman people in 380.


In the fourth century, various theories of the ontology (nature) of Christ competed for domination in the church. Commentators often describe this as a competition between the Nicene Creed and Arianism, but this reflects a superficial understanding of the situation at the time. For example:

(1) The article on the Nicene Creed shows that Christ, in this decree, is subordinate to the Father. And since the term Arianism is often understood to refer to any theory that this is contrary to the Trinity doctrine, in which Christ is co-equal with the Father, the Nicene Creed may also be regarded as Arianism.

(2) The term Arianism comes from the name of Arius, a priest from Alexandria in the fourth century, whose teachings began the entire huge debate over the nature of Christ. During the 50 years after Arius’ teachings were rejected by the Nicene council, various theories of the nature of Christ prevailed that were different from both Arius’s teachings and the Trinity doctrine.

A more precise delineation of these competing theories is as follows:

Same Substance

The Council of Nicene in 325 agreed that Christ is Homoousionof the same substance” as the Father. , was by and later ratified by the First Council of Constantinople (381).

Like the Father

In July 359, the Council of Ariminum concluded that the Son was “like the Father,” without reference to substance. In this view, the Bible does not reveal whether the Son is of the same substance as the Father and we, therefore, should not speculate about such things. See Homoian or Homoeanism.

Similar in Substance

The council of Seleucia agreed in 359 that the Son was “similar in substance” to the Father but not necessarily of the “same substance,” as the Nicene Creed. See Homoiousian or Arian controversy.

Dissimilar Substance

The council of Constantinople in 359 at first accepted that the substance of the Son was “dissimilar” from that of the Father. See Heteroousian or Arian controversy. This is similar to what Arius taught.

One Substance

It is possible to compare this list of theories to the teaching that the Son was “of one substance” with the Father. This is a refinement and further development of the “same substance” concept of the Nicene Creed. While “same substance” may be understood as that the Father and Son have the same substance like people have the same substance, “one substance” implies that they share one common substance; like two Persons in one body or being, as in the Trinity doctrine, where the Father and Son are three Persons in one Being.

Trinity Doctrine

The Nicene Creed (325) and the Creed of Constantinople (381) form the basis of the Trinity Doctrine, but the language of three distinct and infinite hypostases, or divine persons, the Father, the Son, and the Holy Spirit, that possess the very same (numerically the same, as opposed to qualitatively the same) divine ousia, only became universally accepted after the First Council of Constantinople in AD 381. (See, Homoousion – Wikipedia or Lienhard, Joseph T. Ousia and Hypostasis.)

The Trinity Doctrine was only fully formulated in the Athanasian Creed from the late fifth or early sixth century.


The point is this: This article shows that Emperor Theodosius I, when he came to power, crushed Arianism. However, what he really crushed was all resistance to the teaching of the Nicene Creed that the Father and Son have the same ousia.

Summary of this article

Constantine had a decisive influence on the formulation of the Nicene Creed but later rejected the Homoousion Christology of the Nicene Creed. The emperors who succeeded Constantine crushed the church leaders who taught the homoousion principle in the Nicene Creed. When emperor Valens died in 378, the imperial capital was solidly Arian.

Theodosius I succeeded Valens. He was a passionate supporter of Homoousion Christology. Commentators often refer to the Council of Constantinople of 381 as the turning point where Arianism was replaced by Nicene Christology, but that council was a mere formality. Already prior to the council, Theodosius outlawed all other forms of Christianity and exiled Arian bishops (Theodosian Code 16:2, 1 Friell, G., Williams, S., Theodosius: The Empire at Bay, London, 1994 – See, Homoousion – Wikipedia). Furthermore, Arians were not allowed to attend the Council of 381.

The 381 Council, therefore, was simply a formality. The real decisions were taken by the Roman Emperor. Theodosius, with the strong arm of the empire, effectively wiped out Arianism among the ruling class and elite of the Eastern Empire. This supports again the main thesis of this article series, namely that the emperors had a decisive influence on the Christology of the church.

The 381 Creed does not contain the Trinity concept, namely that the Father, Son and Holy Spirit are three Persons with three minds or wills in one Being. But Theodosius’ Edict of Thessalonica of 380 does prescribe Trinitarian theology. In other words, the State laws were Trinitarian while the church decreed lagged behind. This also supports the thesis that the Christology of the church was determined by the emperors.

In the centuries after Theodosius, the church formulated the doctrines that Christ had two separate natures, namely that He had both a divine and a human nature, and that Mary is the Mother of God.

Articles in this series include:

Christology of the persecuted church – first 300 years
Council of Nicaea – A.D. 325 
The Nicene Creed Interpreted
Fourth Century Arianism

What did Arianism believe in the fourth century?
Long Lines Creed – one of the creeds during the Arian period
Death of Arianism among the Roman people – Current Article

The massacres of the Waldensians. – Middle Ages

The Empire was solidly Arian.

As discussed in the article on Fourth Century Arianism, Constantine had a decisive influence on the formulation of the Nicene Creed. He forced the Council of Nicaea to accept the key term Homoousios and to condemn Arianism. However, just a few years later, Constantine reversed his position, banished the main promotor of the Nicene Creed (Athanasius), and allowed the Arian bishops who were exiled after Nicaea, to return. 

Constantine’s son and successor, Constantius II, was openly Arian.  At first, Constantius only ruled in the east but, by the year 353, he became the sole ruler of the empire. He crushed the Nicene party, forcing the western bishops to abandon Athanasius and exiled leaders of the Nicene party.

The next emperor (Julian) did not choose sides, but he ruled only for three years.

Valens (364–378) succeeded Julian and revived Constantius’ anti-Nicene policy. He also exiled Nicene bishops to the other ends of the empire and often used force against them. Consequently, when Valens died in the year 378, the imperial capital of the empire (Constantinople), which by then has existed for 50 years, WAS SOLIDLY ARIAN.

Theodosius wiped Arianism out.

Theodosius I succeeded Valens. He and his wife Flacilla were passionate supporters of the Nicene Creed. Flacilla was instrumental in Theodosius’ campaign to end Arianism.  Sozomen reports an incident where she prevented a meeting between Theodosius and Eunomius of Cyzicus, who served as figurehead of the most radical sect of Arians. Ambrose and Gregory of Nyssa praised her Christian virtues (Roman Catholic Encyclopedia (1909), article “Ælia Flaccilla” by J.P. Kirsch)..

Commentators often refer to the First Council of Constantinople, which Theodosius convened in the spring of 381, as the turning point where Arianism was replaced by Nicene Christology, but that council was a mere formality:

Firstly, Theodosius already on 27 February 380, with the Edict of Thessalonica (see Definition of orthodoxy in Theodosius I) decreed that Homoousian Christianity (the Nicene Creed) will be the only legal religion of the Roman Empire and that Christians teaching contrary views will be punished. By means of this edict, Theodosius outlawed all other forms of Christianity.

Secondly, the incumbent bishop of Constantinople was an Arian. Two days after Theodosius arrived in Constantinople, on 24 November 380, and therefore also prior to the First Council of Constantinople in the spring of 381, he exiled the Arian bishop and appointed Gregory of Nazianzus, the leader of the rather small Nicene community in the city, as bishop over the churches of that city.

Thirdly, only supporters of the Nicene Creed were allowed into the Council of 381. The previous Arian bishop and leaders were already banished and Arians arriving to attend the council were denied admission.

The 381 Council, therefore, was simply a formality. Theodosius, with the strong arm of the empire, effectively wiped out Arianism from the Roman Empire.  

Edict of Thessalonica

This edict states:

According to the apostolic teaching
and the doctrine of the Gospel, let us believe in
the one deity of the Father, the Son and the Holy Spirit,
in equal majesty and in a holy Trinity.

We authorize the followers of this law to assume the title of Catholic Christians; but as for the others, since, in our judgment, they are foolish madmen, we decree that they shall be branded with the ignominious name of heretics, and shall not presume to give to their conventicles (places of worship) the name of churches. They will suffer in the first place the chastisement of the divine condemnation and in the second the punishment of our authority which in accordance with the will of Heaven we shall decide to inflict. — Edict of Thessalonica (Documents of the Christian Church, Henry Bettenson, editor, 1967, p. 22)

The term “Catholic” in this quote means ‘universal’.  The word “Catholic” only became part of the name of the Catholic Church in 1054, at the East-West schism.

Summarized, Church historian Sozomen reports as follows on the Edict of Thessalonica:

Gratian bestowed the government of Illyria and of the Eastern provinces upon Theodosius. The parents of Theodosius were Christians and were attached to the Nicene doctrines. Theodosius made known by law his intention of leading all his subjects to the reception of that faith which was professed by Damasus, bishop of ROME, and by Peter, bishop of ALEXANDRIA. He enacted that the title of “Catholic Church” should be exclusively confined to those who rendered EQUAL HOMAGE to the Three Persons of the Trinity and that those individuals who entertained opposite opinions should be treated as heretics, regarded with contempt, and delivered over to PUNISHMENT. (Sozomen’s Church History VII.4)

The First Council of Constantinople was a mere formality.

It was customary, in the fourth century, for emperors, as the real heads of the church, to appoint church leaders and convene church councils. Similarly, Theodosius convened the First Council of Constantinople in the spring of 381. It is also known as the Second Ecumenical Council. ‘Ecumenical’ means it represents all Christian Churches and perspectives, but that was certainly not the case in this instance:

Theodosius already outlawed Arianism in the previous year, with the threat of punishment for people that teach anything different.

Gregory of Nazianzus—the leader of the Nicene party in the city—presided over part of the Council and vehemently opposed any compromise with the Homoiousians (those who believed that the Son’s substance is “similar” to the Father’s). (Lewis Ayres – Nicaea and its legacy – Oxford University Press. ISBN 978-0-19-875505-0. Retrieved 21 October 2011)

Arians were not admitted into the council. Theodosius already banished the previous Homoiousian bishop and leaders. And 36 Pneumatomachians arrived to attend the council but were denied admission when they refused to accept the Nicene Creed.

Gregory resigned from his office and Nectarius, an unbaptized civil official, was chosen to succeed Gregory as president of the council (Wikipedia, also note 17). Nectarius, as a civil servant, was fully under Theodosius’ control.

The Council, not surprisingly, confirmed Theodosius’ installation of Gregory Nazianzus as Bishop of Constantinople, accepted the Niceno-Constantinopolitan Creed of 381 and dogmatically condemned of all shades of Arianism as heresy. 

Contents of the Creed of 381

The Holy Spirit

The 325 Creed merely mentions the Holy Spirit in connection with the Father and Son. It does not refer to the Holy Spirit as theos (“god” or “God”) or that the Spirit is of the same substance as the Father. 

The 381 Creed goes much further. The 5 words about the Holy Spirit in the Nicene Creed of 325 became 33 words in the creed of Constantinople, saying:

    • That the Holy Ghost is “the Lord and Giver of life,”
    • That He proceeds from the Father and
    • That He is worshiped together with the Father and the Son.

The 381 Creed, therefore, describes the Holy Spirit much clearer as a separate Person and as God.

The HarperCollins Encyclopedia of Catholicism, “God,” p. 568, states that the teaching of the three Cappadocian Fathers “made it possible for the Council of Constantinople (381) to affirm the divinity of the Holy Spirit, which up to that point had nowhere been clearly stated, not even in Scripture.

Note: Catholics are not concerned if their doctrines are not found in the Bible because they believe in continued revelation through the church.

The Trinity

As discussed in the article on the Nicene Creed, the present writer does not find the Trinity concept (namely that the Father, Son and Holy Spirit three Persons with three minds in one Being) in the Nicene Creed. It is also absent from the creed of 381. (See the Comparison between the creed of 325 and 381.) 

However, the Edict of Thessalonica of 380, quoted above, which was an act of law by the emperor, made Trinitarian theology law. Compare the opening phrases of the Edict of Thessalonica of 380:

“Let us believe in
the one deity of the Father, the Son and the Holy Spirit.”

With the opening phrase of the Creed of 381:

“We believe in
one God, the Father Almighty …
And in one Lord Jesus Christ …
And in the Holy Ghost”

An edict which Theodosius issued after the Council of 381 is also clearly Trinitarian:

“We now order that all churches are to be handed over to the bishops who profess Father, Son and Holy Spirit of a single majesty, of the same glory, of one splendour” (quoted by Richard Rubenstein, When Jesus Became God, 1999, p. 223).

In other words, the State laws were Trinitarian while the church creeds lagged behind. The first clear Trinitarian church statement is the Athanasian Creed which was not formulated by a Church Council and originated perhaps 100 years later. The contents of Theodosius’s decrees, when compared to the church decrees, support the main thesis of these articles, namely that the decisions, with respect to which Christology the church will adopt, was made by the emperors; not by ecumenical councils.

Post-381 Trinity Development

Mother of God

Relatively soon after Theodosius crushed Arianism, the church formulated the doctrine of Mary as the Mother of God. Britannica reports:

Nestorius, the patriarch of Constantinople, taught that Mary, the mother of Jesus Christ, may not properly be called the mother of God (Greek Theotokos, or “God-bearer”), because she was the mother only of the human Jesus, not of the preexistent Word of God. The Council of Ephesus (431) condemned this teaching. 

This matter, and its relationship with the Trinity doctrine, has not been further investigated for this article. According to the 381 Creed, the “Lord Jesus Christ” has been “begotten of the Father.” How Mary may the called mother of God and how this relates to the Trinity doctrine has not been investigated by me.

The Two Natured of Christ

The Arians objected to the creeds of 325 and 381 by asking, if the Son is of the same substance as the Father, why did He say that He does not know the day and hour of His return? Why does only the Father know that (Matt 24:36)? And why did Jesus say that He only do and say what the Father gave Him to do and say (e.g. John 5:30; 8:28).  Do such statements not imply that He is subordinate to the Father? 

In response to such questions, the church developed the teaching that Christ had two natures:

This council (of Ephesus) gave rise to monophysitism, which taught that Christ only had one nature. It emphasized Christ’s divine nature to such an extent that it effectively negated Christ’s humanity. It compared the relationship between Christ’s humanity and his divinity to a single grain of sugar in the ocean. Pope Leo I (reigned 440–461) led a reaction against this monophysite doctrine that culminated in the Council of Chalcedon (451). This council concluded that Christ had two distinct natures that were neither commingled nor divided and that were equally present in one person. (Britannica)

See also the Wikipedia article on the Council of Chalcedon

The key word in the quote from Britannica is perhaps “equally.”  The argument is that Christ’s subordination statements in the New Testament must be understood as Him speaking from His human nature. 

Most Christians today accept the dual nature theory. Opponents of this theory point out that this does not solve the problem, but makes it worse, for it means that Jesus was not telling the truth when He said that He does not know, for in His divine nature He actually knew.

Chalcedonian Schism

The decisions at Chalcedon led to the Chalcedonian Schism. The patriarchates of Rome, Constantinople, Alexandria, Antioch and Jerusalem accepted the decisions of the council but the Egyptian (Coptic) and Syrian, Ethiopian, and Armenian Christians rejected the Chalcedonian formula:

They declared that Christ’s human and divine natures, while distinct, were equally present through the mystery of the Incarnation in a single person. (Britannica)

In other words, while the Chalcedonian Creed declared that the two natures “were neither commingled,” the opposing party maintained that divinity and humanity were united in one undivided nature in Jesus Christ.

Athanasian Creed

It was the Athanasian Creed—formulated around the year 500, give or take 50 years—which became the standard formulation of the Trinity theory throughout the middle ages. It is still used today by many denominations in liturgy and confessions. This creed was not written by Athanasius. He died more than a century earlier. Neither was this creed produced by any known church council.

The problem with all previous creeds is that they define the Father alone as God, but then proceed to elevate the Son and the Holy Spirit to the same level as the Father. This may mean that we have three Gods (polytheism), while the Bible is strictly monotheistic. The Athanasian Creed, for the first time, strongly and repeatedly emphasizes the oneness of the Father, Son and Holy Spirit in a single Being. It declares that they together are the one God of the Bible. For example:

So the Father is God;
the Son is God;
and the Holy Ghost is God.
And yet they are not three Gods; but one God