One of the most perplexing and confusingly ambiguous passages in scripture is found at 2 Cor 5.21, where Paul writes:
For our sake [God] made [Christ] to be sin who knew no sin, so that in him we might become the righteousness of God.
The question is rightly asked: what could it mean for the Lord to be made sin, and yet not to know sin? What could Paul be speaking about here?
Typically you will find this text interpreted in evangelical Protestant circles as a reference to Christ's suffering on the cross in punishment for our sins. Christ is "made sin," according to this interpretation, because God imputes to him our sins and he is punished for them. The interpretation is a forensic one, grounded in a legal metaphor of salvation.
My convictions are different, and I much prefer the more ontological interpretation that Maximus gives. He interprets the line in the light of the incarnation of God in Jesus Christ. (My quotations to follow are translated from the Romanian text, found in the Filocalia Sfintelor Nevoințe ale Desăvîrșirii, vol. 3.)
In Questions to Thalassius 42, the distinction is made between the sin of the will and the sin(fulness) of the nature. Because the will of Adam voluntarily turned from what is good to what is evil, it is condemned. Furthermore, this turn affected the whole nature of humanity, which was subsequently deprived of its immortality. The sin of the will refers to the voluntary misdirected action of the will, away from the Good and from God. The sin of the nature refers to the corruptibility inherited by the human nature as due punishment for the sin of the will.
God saw fit to punish the voluntary sin of Adam through a change of his nature from incorruptibility to corruptibility, from immortality to death. Maximus says that God judged that it would not be good for man, who corrupted his free will, to have an immortal nature (42.6). At the same time, God desired to save the nature of man from death and corruptibility, and so assumes a human nature in Christ, fallen as it is, and saves it through his obedience.
Now, Christ does not himself personally sin. There is no voluntary sin in the life of Christ, no misdirected act of the will. Nevertheless Christ does become sin insofar as he takes upon himself a human nature subject to passion and temptation and death, same as the rest of us have.
Maximus distinguishes between "my sin" and "the sin for me," echoing through the latter phrase the terminology used by Paul above in 2 Cor:
The Lord not knowing, therefore, my sin, that is, the misdirection of my free will, he did not take upon himself nor was he made my sin. But taking upon himself that sin for me, in other words, the corruptibility of nature for the sake of the fallenness of my will, he became for our sake a man subject to passions by nature, undoing my sin through the sin for me (42.5).
The narrative, then, is like this. The sinful act of Adam brought corruption and death to human nature. Christ assumes a corrupt and moribund human nature, but does not sin. Because he does not sin but keeps his will obedient to God, he is raised from the dead and his nature is transformed unto immortality and incorruptibility.
The scriptural narrative here is obvious:
In the days of his flesh, Jesus offered up prayers and supplications, with loud cries and tears, to the one who was able to save him from death, and he was heard because of his reverent submission. Although he was a Son, he learned obedience through what he suffered; and having been made perfect, he became the source of eternal salvation for all who obey him (Heb 5.7-9).
And being found in human form, he humbled himself and became obedient to the point of death—even death on a cross. Therefore God also highly exalted him and gave him the name that is above every name (Phil 2.7-9).
For God has done what the law, weakened by the flesh, could not do: by sending his own Son in the likeness of sinful flesh, and to deal with sin, he condemned sin in the flesh (Rom 8.3).
Maximus comments on this passage implicitly when he says:
The transformation of the nature towards passion, towards corruptibility and towards morality is therefore the condemnation of the sin committed by the free will of Adam. Humanity did not have this state from God from the beginning, but brought it about and came to know it voluntarily committing sin through disobedience. The condemnation unto death is the fruit of this sin. The Lord taking therefore this condemnation of my freely chosen sin, that is, the passion, corruptibility, and mortality of my nature, he became sin for my sake through passion, death, and corruptibility, voluntarily clothing himself with my sinful nature -- he who was not condemned by free choice. He did this in order to condemn sin and my condemnation of will and nature, at the same time casting out of my nature sin, passion, corruptibility, and death (42.6).
Through his obedience in a corrupt nature, an obedience which results in his resurrection from the dead and the transformation of his nature to incorruptibility, God condemns sin in the flesh.
I much prefer this incarnational interpretation of Paul's language to the forensic one. The Logos of God assumes a fallen human nature such as ours, and through the obedience of his will, he sanctifies it and redeems it for us all, condemning sin in the flesh by destroying its presence and its effects.