What interests me about those who tend to advocate the latter reading (e.g. N.T. Wright, Ben Witherington III) is that they tend to deny the imputation of Christ's active obedience (IAO). On the other hand, the staunchest defenders of the traditional former rendering, (e.g. D.A. Carson) tend to affirm IAO. Weird! Why so?
Well, firstly, I'm not suggesting that we adapt our exegesis/translations in order to cater to our theological shibboleths. Nevertheless, I'd have thought that those partial to the IAO would see the force in being justified through "the faithfulness of Christ". In my opinion, if the phrase, "pistis Christou" is best translated "faithfulness of Christ" then the arguments for IAO are settled in favour of said doctrine hook, line and sinker.
It seems Michael Horton agrees. In his, excellent, Covenant and Salvation, he writes:
What makes all the difference is whether one is legally incorporated into a promise-covenant or a law-covenant. What is transferred to the believer, therefore, is not the inherent person of God or Christ, but the record of a perfectly acceptable life that has been lived, offered up, received, and raised again for us. It is a verdict declared because of Christ's faithfulness to the covenant....While it is far from nomistic, such a representational view of justification is surely, from beginning to end, covenantal.Powerful stuff. I can't imagine any more debates over IAO if we read Romans 3:22 saying "the righteousness of God is given through the faithfulness of Jesus Christ for all who believe."
...Longenecker thinks that, in Paul's view, Christians participate in Jesus' covenant fidelity by faith. This is why Paul says in [Gal.] 2:16b: 'We believed in Christ Jesus, in order that his covenant faithfulness might be effective for us'" - as also 3:22. "Paul's 'faith' language in these verses is fundamentally language of participation." So justification (i.e. covenant membership) is based for Paul now not on the law but on Jesus' covenant faithfulness, which he receives by faith. His faithfulness is "rubbish"; "only Christ's faithfulness [is]....the mark of his covenant membership before God." (p112)