Commonly, in the last century or two or three, atonement has come to mean making amends or paying a penalty to meet legal demands, to propitiate wrath or to adjust one’s to legal standing. To some atonement is the thing Christ did to reconcile the Father unto us and assuage His offended wrath.
But that is not the original meaning of the term, and it is definitely not the meaning of the word in the Bible. The only place you’ll find the word, in the King James Version, is in Romans 5:10. But the word in the Greek is the very common word ‘katallasso’. There’s no hint of making amends in this word. It means ‘reconciliation’. Holman’s Bible dictionary defines this word as follows:
Reconciliation … specifically the reconciliation between God and humanity effected by the death, burial, and resurrection of Christ. … The New Testament rarely uses a word for atonement. The basic Greek word is usually translated “to reconcile”. The basic meaning is to establish friendship.
Therefore Romans 5:10, in the NASB, reads:
“For if while we were enemies we were reconciled to God through the death of His Son, much more, having been reconciled, we shall be saved by His life” (Rom 5:10).
The dictionaries agree that this word ‘atonement’ is a made-up word, namely ‘at-one‐ment’. That’s the way the word started. It was based on a verb, ‘to one’. Two people are fighting, and you are sent out to ‘one’ them. Not ‘win’ them; to ‘one’ them. And then when you have succeeded in ‘one-ing’ people, then, hopefully, they would remain in a state of oneness. It is the state of being ‘at one’ that is atonement, not the process ‘one-ing’ people. Atonement therefore means to be in harmony or unity. That is what “atonement” meant when the Bible was first translated into English. In the Bible it is God, because He loves us, that sent His Son (“the Lamb of God”-John 1:29) to bring His people back to Him (John 3:16).
There’s only one dictionary that really give the history of the word, and that’s a multi-volume Oxford English Dictionary. It shows how, for a long time, it was used in its original sense of being at one, reconciling people to harmony; friendship is often mentioned, unity, and so on. Now, later on somehow, it was changed to mean ‘making amends, paying penalty’, and that’s the way it’s commonly used now.
So how did the meaning of the word atonement change so much? It was the forensic doctrine of salvation that changed the meaning of “atonement” over the centuries. The forensic doctrine of salvation teaches that somebody must pay for sins committed. TIt presents God as angry and the death of Christ as a sacrifice to pacify God. It was because the reformers had this understanding of the purpose of Christ’s death that the meaning of “atonement” has slowly changed over the centuries to “reparation for an offence or injury” (Merriam-Webster).
But that is not how we should understand the purpose of Christ’s death. It is not God that must change. The blood of the Cross did not change how the Father feels about sinners. The opposite is rather true, namely that the blood of Christ was the means by which the Father changed the hearts and minds of His creatures; to be reconciled to Himself (Colossians 1:20). It is us that must change. It is not God that is angry; it is His creatures that are “hostile in mind” (Colossians 1:21). In the Bible God is never reconciled to us. Colossians 1:20-22 indicates that God, through Christ, reconciled all things “to Himself” (1:20).
Please also see the article Christ’s death reconciled us to God for further information.