One thought that didn't make the lecture on the atonement (because I ran out of space and didn't have time to work it out fully) is about the righteousness of God in 2 Corinthians 5:21. If this phrase means God's covenant faithfulness/saving action everywhere else it's used in Paul - principally in Romans and Galatians - is that what it means here? If it is, how can we become the covenant faithfulness of God as Paul suggests?
I haven't had a chance to explore this thought fully nor to read the N T Wright paper that I suspect might argue a version of it, but I've been wondering whether since Paul is talking about his ministry in 2 Corinthians 5:11-6:2, a ministry of reconciliation in partnership with God who has himself engaged in such a ministry in Christ, does the exchange of our sin for his righteousness mean that we too become messengers of reconciliation at that moment.
Paul starts his discussion proper 5:14 by pointing out that one died for all and those who live as a result no longer live for themselves but for Christ (v15). He then talks about experiencing new creation through being reconciled to God - something that happens as a result of the ministry of reconciliation that has been entrusted to the likes of Paul and his team - and by implication to anyone who is a new creation (16-18).
He then tells us that it was God working in Christ who brought about the reconciliation that we enjoy - that is that God is working according to his covenant faithfulness, his promise to Abraham that through his family all the nations of the earth would be blessed; in short that he is revealing his righteousness in the gospel exactly as Paul says in Romans 1:16-17 and 3:20f.
God co-opts us as ambassadors so he can make his appeal to the world through us (v20). And that appeal is that through faith in Christ we swap our sin - our state of being unreconciled and of being an ambassador or spokesman for non-reconciliation (as the Corinthians were turning out to be!) - for his righteousness, his saving activity in the world.
In 6:1 (remember there were no chapter divisions in the original letter!) he reiterates that he's working in partnership in urging the Corinthians to accept the grace of God for there's no time like the present (to paraphrase 6:2)..
Of course this in no way suggests that the cross is not an exchange of sin for right standing before God (though that is not the way Paul uses the term righteousness). He doesn't need to say it in v21 since he's already made it abundantly clear in v14-17 that it is by Christ's death for all that we are made new creations; the cross has brought about the turning of the ages. and if we are new creations, our sin (all that made up the old of v17) must have been dealt with by the action that turned the ages, the cross.
And the language of v21 is itself deeply substitutionary. The exchange of sin for righteousness happens because of a prior exchange. But note the exchange that Christ made on the cross was not his righteousness for sin. That's not what Paul says. He says that Christ exchanged his sinlessness for sin. Righteousness in paul is a term reserved for God's action in making good his promise to Abraham that he'd bless the nations through his seed (Galatians 3). the amazing thing is that as we put of faith in Christ and become new creatures in him so we swap our sin for a partnership with God in helping to put things right; we cease to be part of the problem and become part of the solution, in that we become messengers of God reconciling good news.
Does that make sense? Or have I missed something vital?