... This passage [2 Cor. 5:21] teaches that Christ's participation in human sin enables humans to participate in the righteousness of God through union with Christ. The word ginomai ("become") is not a synonym for logizomai ("count"). So Paul does not say that "God imputed our sin to the sinless one, and imputed God's righteousness to us." We can say what the text says, no more and no less: Christ was made sin probably in the sense of carrying, bearing and taking sins upon himself, and those are in Christ share in the "righteousness of God."
[Michael F. Bird, "Progressive Reformed View," in James K. Beiby & Paul Rhodes Eddy, eds., Justification: Five Views (Downers Grove, IL: IVP Academic), 149]
τὸν μὴ γνόντα ἁμαρτίαν ὑπὲρ ἡμῶν ἁμαρτίαν ἐποίησεν, ἵνα ἡμεῖς γενώμεθα δικαιοσύνη θεοῦ ἐν αὐτῷ. (2 Cor. 5:21)
He made the one who knew not sin to be sin for our sake, in order that we might become the righteousness of God in him. (2 Cor. 5:21 Own translation)
The attack against the very idea of Christ's active righteousness continues. The so-called "Progressive Reformed" view (certainly "progressive" but not Reformed) joins with the New Perspective on Paul and the Federal Vision in denying the very idea that Christ has an active righteousness which is imputed to believers. Rather, "Progressive Reformed" proponent Micheal F. Bird proposes that our faith is counted as righteousness before God, with 'righteousness' being understood as a status before God not a property transferred from Christ to believers (pp. 147-8).
In reducing all things to relational categories, Bird likewise does the same in his interpretation of 2 Cor. 5:21. For Bird, 2 Cor. 5:21 shows Christ to be "carrying, bearing and taking sins upon himself" and we "share in the 'righteousness of God.'" However, is that a viable interpretation of the text?
We note there in 2 Cor. 5:21, the two clauses are parallel to each other. Therefore, however one wants to interpret the action of "making sin," the exact same interpretation is to be applied to the action of us "becoming." Bird's semantic argument is discounted as nobody is saying that λογιζομαι is the same as γινομαι. The issue has always been the interpretation of this verse, not merely the form of this verse.
The traditional interpretation has been that Christ was imputed our sins, and therefore in the same manner Christ's righteousness was imputed to us, thus preserving the parallelism. If we adopt Bird's proposed alternate interpretation, then the question is asked, "In what sense does Christ carry and bear our sins?" If Christ actually became our sin, then that would mean that Christ was actually sinful on the cross, which is a heretical notion. If Christ merely died to pay the penalty for sin but did not actually carry sin, just its penalty, then to preserve the parallelism, we must also say that believers do not actually become the righteousness of God, but merely enjoy the rewards of righteousness. To throw the word 'union' around is vacuous, for what kind of 'union' are we talking about? If we partake of the 'righteousness of God' via 'union,' then does that therefore make Christ sinful as He partake of our sins via 'union'?
This type of sloppy exegesis by Bird is sad. If speaks of a failure to think through the logical conclusions of one's interpretation. Was Christ actually sinful at the Cross when He bore the sins of the world? How does Bird's language of "Christ's participation in human sin" and his interpretation of 2 Cor. 5:21 not leave him open to the charge of denying that Christ has always been sinless?
2 Cor. 5:21 has always been one of the key verses used to prove the imputation of Christ's righteousness to us. It is so not because λογιζομαι or any of its cognates is found in the verse, but rather that such is the best interpretation of the verse in light of its grammatical form and coherence with other aspects of Systematic Theology.