It is a common argument among modern Eastern Orthodox, laymen and apologists alike, to argue that there was no concept of an atonement (specifically the substitutionary atonement) for sins until the time of the Church Father Anselm, or (as some put it) at least by the tenth or eleventh century (around the time that Anselm lived). I covered this extensively in my podcast episode where I reviewed an audio of Eastern Orthodox apologist Frederica Mathewes-Green, who often repeats this contention in her writings and lectures.
However, the idea that the atonement was a foreign concept within Patristics is patently false. Below are some quotes from Church Fathers regarding this topic. I will most likely be adding to this list as research continues.
“Let us reverence the Lord Jesus Christ, whose blood was given for us…” [Clement of Rome; First Epistle, Ch. 21]
“On account of the Love he bore us, Jesus Christ our Lord gave His blood for us by the will of God; His flesh for our flesh, and His soul for our souls.” [ibid; Ch. 49]
“He Himself took on Him the burden of our iniquities, He gave His own Son as a ransom for us, the holy One for transgressors, the blameless One for the wicked, the righteous One for the unrighteous, the incorruptible One for the corruptible, the immortal One for them that are mortal. For what other thing was capable of covering our sins than His righteousness? By what other one was it possible that we, the wicked and ungodly, could be justified, than by the only Son of God? O sweet exchange! O unsearchable operation! O benefits surpassing all expectation! that the wickedness of many should be hid in a single righteous One, and that the righteousness of One should justify many transgressors!” [Methetes; Epistle to Diognetus, Ch. 9]
“But again, showing that Christ did suffer, and was Himself the Son of God, who died for us, and redeemed us with His blood at the time appointed beforehand, he says: ‘For how is it, that Christ, when we were yet without strength, in due time died for the ungodly? But God commendeth His love towards us, in that, while we were yet sinners, Christ died for us. Much more, then, being now justified by His blood, we shall be saved from wrath through Him. For if, when we were enemies, we were reconciled to God by the death of His Son; much more, being reconciled, we shall be saved by His life.’” [Irenaeus; Against Heresies, Book III, Ch. 16, Section 9]
“We were enemies of God by means of Sin; and God ordained that the sinner should die. Of two things, then, one must needs have happened; either that God should adhere to His word, and destroy all men, or that by giving scope to His benignity He should annul His sentence. But see the wisdom of God. He secured, at once, reality for His sentence, and active operation for His benignity. Christ ‘took on Himself our sins in His body, on the Tree, that we, being dead to sins’ through His death, ‘should live unto righteousness.’ He that died for our sakes was not of small account. He was not a literal sheep, He was not a mere man, He was not simply an Angel, but He was God Incarnate. The iniquity of the sinners was not so great as was the righteousness of Him that died for them. Our sins did not equal the amount of His righteousness, who laid down His life for us, who laid it down when He pleased, and when He pleased resumed it.” [Cyril of Jerusalem; Lecture 13, 53]