St Maximus the Confessor commentary on 2 Corinthians 5:21

ST. MAXIMUS THE CONFESOR
AD THALASSIUM 42
On Jesus Christ, the New Adam Who "Became Sin"

Q. [285] How is it that we are said to commit sin and know it (cf 1 In 1:8), while the Lord became sin but did not know it? How is it not more serious to become sin and not know it, than to commit sin and know it? For the Scripture says, For our sake God made him become sin who knew no sin (2 Cor 5:21).

R. Having originally been corrupted from its natural design, Adam's free choice corrupted along with it our human nature, which forfeited the grace of impassibility. Thus came sin into existence. The first sin, culpable indeed, was the fall of free choice from good into evil; the second, following upon the first, was the innocent transformation of human nature from incorruption into corruption. For our forefather Adam committed two "sins" by his transgression of God's commandment: the first "sin" was culpable, when his free choice willfully rejected the good; but the second "sin," occasioned by the first, was innocent, since human nature unwillingly put off its incorruption. Therefore our Lord and God, rectifying this reciprocal corruption and alteration of our human nature by taking on the whole of our nature, even had in his assumed nature the liability to passions which, in his own exercise of free choice, he adorned with incorruptibility. And it is by virtue of his assumption of this natural passibility that he became sin for our sake, though he did not know any deliberate sin because of the immutability of his free choice.' Because his free choice was incorruptible, he rectified our nature's liability to passions and turned the end of our nature's passibility—which is death—into the beginning of our natural transformation to incorruption. In turn, just as through one man, who turned voluntarily from the good, the human nature was changed from incorruption to corruption to the detriment of all humanity, so too through one man, Jesus Christ, who did not voluntarily turn from the good, [287] human nature underwent a restoration from corruption to incorruption for the benefit of all humanity.

Therefore the Lord did not know "my sin", that is, the mutability of my free choice. Neither did he assume nor become my sin. Rather, he became the "sin that I caused"; in other words, he assumed the corruption of human nature that was a consequence of the mutability of my free choice. For our sake he became a human being naturally liable to passions, and used the "sin" that I caused to destroy the "sin" that I commit. Just as in Adam, with his own act of freely choosing evil, the common glory of human nature, incorruption, was robbed—since God judged that it was not right for humanity, having abused free choice, to have an immortal nature—so too in Christ, with his own act of freely choosing the good, the common scourge of our whole nature, corruption, was taken away. At the resurrection of Christ, human nature was transformed into incorruption because his free choice was immutable. For God judged that it was right for man, when he did not subvert his free choice, once again to recover an immortal nature. By "man" here I mean the incarnate Logos in virtue of the fact that he united to himself, hypostatically, the flesh animated by a rational soul. For if the deviance' of free choice introduced passibility, corruptibility, and mortality in Adam's nature, it only followed that in Christ, the immutability of free choice, realized through his resurrection, introduced natural impassibility, incorruptibility, and immortality.

Hence the mutation of human nature over to passibility, corruption, and death is the condemnation of Adam's deliberate sin. Man was not created by God in the beginning with such a corrupted nature; rather, man invented and knew it since he created deliberate sin through his disobedience. And clearly condemnation by death is the result of such sin. Yet the Lord took on this very condemnation of my deliberate sin, that is to say, the passibility, corruptibility, and mortality of our nature. [289] He became the "sin" that I caused, in terms of the passibility, corruptibility, and mortality, and he submitted voluntarily to the condemnation owed me in my nature, even though he himself was blameless in his freedom of choice, in order to condemn both my deliberate "sin" and the "sin" that befell my nature. Accordingly he has driven sin, passion, corruption, and death from human nature, and the economy of Christ's philanthropy on my behalf has become for me, one fallen through disobedience, a new mystery. For the sake of my salvation, Christ, through his own death, voluntarily made my condemnation his own, thereby granting me restoration to immortality.

In many ways, I think, it has been shown in this brief discussion both how the Lord became sin but did not know it, and how humanity did not become sin but did commit and know sin—both the deliberate "sin" which man committed first, and the subsequent natural "sin" to which the Lord submitted himself on humanity's account, even when he was completely free of the first kind of sin. So according to the intended purpose of the text as we have rendered it here, and respecting the proper conceptual distinction between the two meanings of "sin," it is by no means better to commit and to know sin than to become sin. For the former "sin" incurs separation from God, since free choice voluntarily rejects divine things; but the latter "sin" may very well hinder evil, since it does not allow that wickedness of free choice that is based on the infirmity of nature to advance into concrete action.
(translated by Paul M. Blowers and Robert Lewis Wilken, On the Cosmic Mystery of Jesus Christ, pp. 119-122)