The Rise and Fall of the Homoiousianism


After Nicaea, the ‘Arian’ Controversy raged for another 55 years. During that period, ‘Arianism’ dominated the church. But ‘Arianism’ consisted of several strands. This article explains the theology of the Homoiousians, which was one of those strands. Homoiousian means ‘similar substance’ and was used to say that the Son’s substance is similar to the Father’s.


This article series is largely based on two books:

RH = Bishop RPC Hanson
The Search for the Christian Doctrine of God –

The Arian Controversy 318-381, 1987

LA = Lewis Ayres
Nicaea and its legacy, 2004

Ayres is a Professor of Catholic and Historical Theology

A Compromise

It is often claimed that Homo-i-ousianism (similar substance theology) arose as an attempt to reconcile two opposing teachings, namely: and Homo-ianism:

Homo-ousianism, which comes from the word homo-ousios in the Nicene Creed of the year 325. It means “same substance” and was used to say that the Son’s substance is the ‘same’ (ὁμός, homós) as the Father’s.  If the Son’s substance is the same as the Father’s, then the Son must be co-equal and co-eternal with the Father.

Homo-ianism, on the other hand, refused to use the term substance (οὐσία, ousía). It believes that the Son is “like” (ὅμοιος, hómoios) the Father but subordinate to Him.

It is then proposed that similar substance theology (Homo-i-ousian) was an attempt to reconcile the same substance theology (Homo-ousian) theology with the Homoian notion of similarity. For example, “Gwatkin described the group as a ‘Semi-Arian position modified by an Athanasian influence.” (RH, 349) (Athanasius was the great defender of the same substance theology.)

A Persistent Strand

However, recent scholarship does not accept that Homo-i-ousianism was an attempt to reconcile the two other theologies. Homo-i-ousianism was “most prominently associated with … Basil of Ancyra” (RH, 349) and “the term homoiousios plays no role in Basil’s surviving texts” (LA, 150). This implies that such a compromise was not the purpose. More recently, Lewis Ayres proposed that Homo-i-ousianism was not merely a compromise but “a significant and persistent strand in earlier eastern theology.” (LA, 150)

There are indications that this theology was a restatement or development of the theology of Eusebius of Caesarea, as stated in the letter he wrote to his home church after the Nicene Council, to explain why he accepted that Creed:

Ritter described Homoiousianism “as the right wing of the Eusebian party.” (RH, 349)

“Basil … prefers the term ‘image of the ousia’ to define the Son’s relationship to the Father; it is worth noting that this term was favoured by Eusebius of Caesarea … and also is found in the Second (‘Dedication’) Creed of Antioch 341.” (RH, 353)

Eusebius was “universally acknowledged to be the most scholarly bishop of his day.” (RH, 46) Eusebius was the most influential theologian present at the Council of Nicaea in AD 325.

A Response to Neo-Arianism

Both Lewis Ayres and R.P.C. Hanson stated that the formulation of Homoiousian theology in 358 by a council of bishops called by Basil of Ancyra was a response to, what Ayres calls, “the emerging shape of Heterousian theology” in the form of the creed of “Sirmium 357,” which was based on the teachings of Aetius. Hanson refers to this as “Neo-Arianism” and as “a new and radical theology” that appears for the first time in the extant ancient records in the form of the “Second Creed of Sirmium of 357,” afterward approved by a larger synod at Antioch (probably in AD 358). ‘Neo-Arianism’ may be an appropriate name because it was “a development” of Arius’ theology. (RH, 348; LA, 149-150)

“Basil’s council sent a delegation to the Emperor Constantius … and this embassy met with success.” The Emperor condemned “Aetius and his teaching” and exiled Aetius and his supporters. This supports the view that this formulation of homo-i-ousianism was particularly intended to oppose the Neo-Arians. (LA, 152-153)

Homo-i-ousian Theology

“The statement which emerged from this council … marks the emergence of a new and coherent theological point of view. This is the theology of those whom Epiphanius, quite undeservedly, calls ‘Semi-Arians’, but who are usually today thought of as Homoiousians, a designation which is more accurate.” (RH, 348-9)

This statement was written by Basil of Ancyra himself (LA, 150) and “is of the highest importance for an understanding of Homoiousian theology.” (RH, 350) It includes “nineteen anathemas which reveal more clearly the position which Basil is attacking.” (RH, 355)

Against Homoian Theology

Homoianism was a dominant Christology during the mid-fourth century. For example, the creeds of the councils of Sirmium in 358, Ariminum in 359, and the key council at Constantinople in 359 / 360 were homoian. It refused to use ousia (substance) language in the formulation of any statement of faith because the Bible does not say anything about God’s substance. Against them, Basil insisted that substance language is necessary to reflect the closeness of the Father and Son expressed by the concepts “Father/Son” and “begotten.” He wrote:

“God must be both Father and creator” (of His Son) (RH, 353). “If we remove this resemblance of ousia,” the Son is merely a created being; “not a Son.” (RH, 353, 354)

Since human sons are like their fathers, the Son of God is like His Father (RH, 352). “The salient irreducible element” in a father/son relationship is “the begetting of a living being that is like in ousia.” (RH, 352-3)

“If the Father gives the Son to have life in himself (John 5:26) … then the Son must have the same life and thus have ‘everything according to essence and absolutely as does the Father’.” (LA, 152)“

Against Homoousian Theology

It is often claimed that the term homo-ousios in the Nicene Creed means “one substance,” namely, that the substance of the Son is one and the same as the Father’s substance. It is on this basis alone that we can argue that the Son is co-eternal and co-equal with the Father. However:

Hanson concluded that “we can … be pretty sure that homoousios was not intended to express the numerical identity of the Father and the Son.” (RH, 202)

Philip Schaff stated: “The term homoousion … differs from monoousion. … and signifies not numerical identity, but equality of essence or community of nature among several beings. It is clearly used thus in the Chalcedonian symbol, where it is said that Christ is “homoousios with the Father as touching the Godhead, and homoousios with us [and yet individually distinct from us] as touching the manhood.”1Philip Schaff, History of the Church volume 3 (Grand Rapids, MI: Eerdmans Publishing Co., 1981 edition) pp.672-673.

The idea of “one substance,” therefore, developed later. In the Homo-ousianism of the Nicene Creed, the Son’s substance is identical with the Father’s, meaning two substances that are identical. 

This is based on a material interpretation of the terms “Father,” Son,” and “only-begotten,” as if God has a body and bodily gave existence to the Son, comparable to how human sons are brought forth. Consequently, the Son’s substance is identical to the Father’s. For example:

“Anathema 13 links the error of thinking of the Father/Son relationship in corporeal terms with that of making the Son identical with the Father.” (RH, 356)

Homo-i-ousianism did not accept this notion. (RH, 352-3) In Homo-i-ousianism, the Son is subordinate to the Father. (RH, 355) If this is true, then the Son’s substance cannot be identical to the Father’s. Basil explained:

“The Son is like the Father in ousia but not identical with him.” (RH, 352-3). 

“As He … was in the likeness of men (John 1:14) … yet not a man in all respects;” “not identical with human nature,” for example. He was not born through natural conception, “so the Son … is God in that he is Son of God,” was “in the form of God,” and is “equal to God (Phil 2:6, 7),” “but not identical with the God and Father.” (RH, 354)

Anathema 13 “damns him who declares … that the Son is identical with the Father … This is manifestly directed against N (the Nicene Creed).” (RH, 355)

Against Sabellian Theology

In Sabellianism, the Son is not a distinct Person. Rather, the Father and Son are parts of one Person. Basil responded:

“This argument that God must be both Father and creator and that the likeness in ousia is necessary … as a safeguard against Sabellianism: that which is like can never be the same as that to which it is like’.” (RH, 353)

The anathemas also attack the apparent Sabellianism of Marcellus of Ancyra. (RH, 355)

Against Neo-Arian Theology

In Neo-Arianism, which was “a new and radical” (RH, 348) adaptation of Arius’ theology, the terms “Father,” Son,” and “only-begotten” symbolize that the Son is the very image of the Father, but not in a corporeal (material) sense. For that reason, in this view, “the Son is ‘unlike(anhomoios) in ousia to the Father” Ayres refers to this as “Heterousian (different substance) theology.” (LA, 149) For example, Basil’s “Anathema 12 strikes him who declares that the Son’s likeness to the Father consists in power but not in ousia.” (RH, 355)

Homo-i-ousianism was somewhere between the Homoousian (same substance) view and the Neo-Arian (different substance) view. 

The End of this Theology

“In AD 359 Constantius decided to emulate his father’s action in calling Nicaea and summon a general council. … A small group of bishops met at Sirmium to draw up a draft creed for discussion. Those present included not only Basil, but also some who were far more suspicious of ousia language. The creed on which they finally agreed … asserts that all ousia language should be avoided. … … Thus, although Basil of Ancyra was influential with the imperial authorities at one point during 358–9, it was not for long, and he never seems fully to have overcome long-standing Homoian influence at court. (LA, 157-8)

Constantius was becoming somewhat hostile to the influence of all of the new movements which had sprung up after the Nicene council. The result was that the Homoiousians disappeared from the stage of history and the struggle to define Church dogma became a two-sided battle between the Homo-ousians and the Homo-ians.


The 55 years of Controversy after the Nicene Creed of 325 revolved specifically around the word homoousios. Since, in the Nicene Creed, this term was an interpretation of the term “begotten,” the differences between the various Christological views are essentially different interpretations of the terms “Father,” “Son,” and “only-begotten.” These interpretations result in different views with respect to the substance of the Son, on the basis of which the five views may be summarized:

      • Sabellianism = One and the same substance
      • Homoousian = Distinct but identical substance
      • Homoiousian = Similar in substance
      • Neo-Arianism or Heteroousians = Unlike in substance
      • Homo-ianism refuses to refer to substance.

Other Articles

  • 1
    Philip Schaff, History of the Church volume 3 (Grand Rapids, MI: Eerdmans Publishing Co., 1981 edition) pp.672-673.

Emperor Theodosius eliminated Arianism from the Roman Empire.


In the fourth century, various theories of the ontology (the nature and relations of being – Merriam-Webster) of Christ competed for domination in the church. Commentators often describe this as a dispute between the Nicene Creed and Arianism, but this reflects a superficial understanding of the situation. For example:

(1) The article on the Nicene Creed lists several indications that it describes Christ as subordinate to the Father. For example, it describes the Father alone as Almighty. In the Trinity doctrine, Christ is co-equal with the Father. Since the term ‘Arianism’ is often used to refer to any theory that is contrary to the Trinity doctrine, if the Nicene Creed describes the Son as subordinate to the Father, then that 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 dispute with his bishop Alexander sparked the Arian Controversy. During the 55 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. This was later ratified by the First Council of Constantinople (381).

No mention of Substance

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 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 per the Nicene Creed. See Homoi-ousian or Arian Controversy.

Different Substance

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

One Substance

Sometimes homoousios (same substance) is translated as “one substance.” Literally, homoousios means “same substance.” Therefore, it may be understood as saying that the Father and Son have the same substance like people have the same substance. In contrast, “one substance” implies that they share one common substance; like two Persons sharing one body or being, as in the Trinity doctrine, where the Father and Son are three Persons but one Being. “One substance,” therefore, is a Trinitarian translation of homoousios.

“According to an anonymous Expositio fidei, in the fourth century the Sabellians made use of the more specific term monoousios, no longer of homoousios, the word which in the meanwhile had become the flag of the Nicene party.”1Ps.-Athanasius, Exp. fid. 2 (PG 25, 204 A) Perhaps monoousios could be translated as “one substance.” 

Trinity Doctrine

The Nicene Creed (AD 325) forms the basis of the Trinity Doctrine, but the language of three distinct and infinite hypostases (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, was only developed in the period 360-380 by the three Cappadocians (See, Pro-Nicene) and only became universally accepted after the First Council of Constantinople in AD 381. (See, Homoousion – Wikipedia or Lienhard, Joseph T. Ousia and Hypostasis.)


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.


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.2Theodosian 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.

Since the 381 Council 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.


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 entire 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 versions 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 this 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

  • 1
    Ps.-Athanasius, Exp. fid. 2 (PG 25, 204 A)
  • 2
    Theodosian Code 16:2, 1 Friell, G., Williams, S., Theodosius: The Empire at Bay, London, 1994 – See, Homoousion – Wikipedia