The doctrine of the imputation of Christ's righteousness is one of those unfashionable doctrines these days. N.T Wright doesn't like it, Bob Gundry doesn't like it, Ben Witherington III doesn't like it,...I could go on. You can't pass righteousness from one party to another like gas passing accross a room (as N.T Wright argues). The number of exegetical heavyweights who are rejecting the doctrine can leave one with the impression that to persist in holding to it, is akin to the stubborn traditionalism of some Roman Catholic exegetes. Nevertheless, I firmly believe that it is a doctrine not only full of comfort for the conscience but also a doctrine that rests on a carefully nuanced and sophisticated exegetical base. D.A Carson, in response to Gundry, explains.
I cannot too strongly emphasize how often Paul's justification language is tied to "in Christ" or "in him" language - yet this brute fact, far from clarifying matters, has sometimes merely muddied the watters.
On the one hand, justification is, in Paul, irrefragably tied to our incorporation into Christ, to our union with Christ. Thus, as we have seen, in Phillipians 3:8-9 Paul wants to be found in him, not having a righteousness of his own. In 2 Corinthians 5:19-21, we are told that God made Christ who had no sin to be sin for us, so that in him we might become the righteousness of God. It is because of God that we are in Christ Jesus, who has become for us righteousness (1 Corinthians 1:30). Passage after passage in Paul runs down the same track. If we speak of justification or of imputation...apart from a grasp of this incorporation into Christ, we will constantly be in danger of contemplating some sort of transfer apart from being included in Christ, apart from union with Christ. (D.A Carson, Justification - What's at Stake in the Current Debates, ed. Husbands and Treier, p72)