5:21 who did not know sin Paul affirms that Christ did not sin, though He was tempted (Matt 4:1–11; Mark 1:12–13; Luke 4:1–13; Heb 4:15).
to be sin This describes how God regarded Christ as sin for the sake of undeserving sinners (compare Gal 3:13). More specifically, Paul may be presenting Christ as a substitute for sinful humanity or he could be referring to Christ’s identification with sin through His union with sinful humanity. Another possibility is that Paul is interpreting Christ’s sacrifice in light of ot sacrificial concepts (e.g., Lev 4:24; 5:12; Isa 53:10).
the righteousness of God Through Christ’s death and resurrection, God demonstrated His righteousness (dikaiosynē) by judging sin yet showing mercy to sinners. Here Paul refers to the idea of Christians becoming the righteousness of God. He may mean that believers, as a result of God’s justification, receive a right standing before God while Christ takes on their sins (Rom 5:8). Alternatively, Paul could be describing God’s righteous character, which believers receive and should live out in their lives. See Rom 1:17 and note.
Barry, J. D., Mangum, D., Brown, D. R., Heiser, M. S., Custis, M., Ritzema, E., … Bomar, D. (2012, 2016).
Faithlife Study Bible (2 Co 5:21). Bellingham, WA: Lexham Press.
The famous verse 21 is often misunderstood, too. It isn’t a general statement about the meaning of the cross, though no doubt Paul would be happy to read it that way as well. It is a statement, as the whole of the last three chapters have been, about his own ministry. He has been called not just to speak about the fact that God has been faithful to the covenant; he is called to embody that faithfulness, to have it worked out, as he has been arguing in chapters 4 and 5, in his own ‘death’ and new life, in his own getting ready to stand before the Messiah’s judgment seat, and above all in his own answering love and devotion to the Messiah who had loved him so much. The cross itself, in all its inexhaustible meaning, stands behind the ministry which Paul exercises, which he wants the Corinthians to understand.
But how is this possible? As he asked in 2:16, who is capable of being God’s agent in this extraordinary work? The answer is in the cross, on which God made the sinless Messiah to ‘be sin’ on our behalf. All our sins, our failings, our inadequacies, were somehow dealt with there, so that we—the apostles, and all who are called to be ‘ministers of reconciliation’—could embody in our own lives the faithfulness of God. No wonder the Corinthians found it difficult to grasp what Paul was up to, why his ministry took the shape it did. Nothing like this had ever been thought of in the world before.
But on this basis he turns to them in the first two verses of chapter 6 with a direct appeal, which comes to us as much as to them. You’ve accepted God’s grace; don’t let it go for nothing! Make the most of it! The new creation is already here. God is saying ‘Yes!’ to all the prophecies and promises (1:20), and he’s saying it right now. This is the day of salvation, the right time. Make the most of it.
Wright, T. (2004).
Paul for Everyone: 2 Corinthians (pp. 66–67). London: Society for Promoting Christian Knowledge.