The strongest argument in favor of imputed righteousness we may infer from the dozens of references to being "in Christ." This union with Christ, which is all over Revelation, the OT, in the "in Christ" references, and explicit in Eph 5. This is obviously what Augustine seized upon in his exegesis on 2 Cor 5:21. If we are in Christ, then we are literally seen as Christ upon judgment, because we are a corporate entity, the very body of Christ with Him as our head.
Let me try to give you an argument.
I am in fact willing to speak of Christ's righteousness imputed to us, as long as you don't try to read it into passage where it isn't present.
Many Reformed seem to think that the point of Christ’s death was to let God forgive our sins. In my view, there’s no problem with God forgiving sins. The OT and Jesus’ own teachings say that God is happy to forgive sins if we, as his followers, are repentant. If you search the Bible, OT and NT, for uses of righteousness applied to humans, you’ll find plenty of them. And in general they do not refer to people who are sinless. They refer to people who are followers of God, who care about other people, and who repent when they sin. Righteous means living as God expects, but specifically living as a follower of God.
Paul teaches justification by faith. But faith unites us with Christ. It makes us one of his people. Thus it makes us righteous. But is that Christ’s righteousness imputed? I think so. We can argue about how people in the OT and current members of other religions might be saved. But we are justified because we are followers of Christ, who act as his servants. But we’re not righteous as isolated individuals. We are righteous as Christ’s followers. He died to establish the new covenant. But covenants constitute a people. Jesus, as the Son of God, is righteous in himself. But we aren’t; we’re righteous only as his followers. (Remember, I think the essence of being righteous is being a follower of God; for us, that means a follower of Christ.) The reason faith in Christ justifies us is because it makes us one of Christ’s people, and we share in Christ’s righteousness, the status he has before God as his Son.
Although Paul never quite says this, I think it’s consistent with what he says.
The problem with White’s reading of this passage is that it doesn’t see the big picture. Christ became sin to save us, through the double exchange described in Rom 6. But that’s not the point Paul is making *here.* Here he’s going beyond that. Christ became sin not just to save us as individuals, but to make us part of God’s own righteousness, his commitment to redeem the world. If you look at the context, you can see that that’s the point he’s making here. Otherwise 5:21 becomes a non-sequitur.
In fact it’s quite likely that the “we” in 5:21 is specifically the apostles. In 5:16 through 6:1 Paul is probably speaking of his own role. God is appealing to the Corinthians through him, and in 6:1 Paul is working as a partner with God (Christ?) — using the translation “as we work together with him.” It is Paul specifically who has become the agent of God’s righteousness to work with the Corinthians. But I think it actually applies to all Christians. We're all Christ's ambassadors, even if that's not the point Paul was making at this point.