…So God’s imputation, in Gundry’s view, is not crediting an external, divine righteousness to Abraham, but counting something that he has, namely faith, to be his righteousness.
What seems out of sync with this interpretation is that Paul’s exposition of imputation, which immediately follows verse 3, gives us a conceptual framework for imputation very different from the one Gundry sees in verse 3. Paul speaks immediately in terms of something external (a wage) being credited to our account, rather than something internal (faith) being treated as righteousness. “Now to the one who works, his wage is not credited according to grace, but according to debt.” If Paul’s conceptual framework were the same as Gundry’s, and verse 3 implied to Paul that the credited righteousness consists of faith, then why would it enter Paul’s mind to illustrate this with the words, “To the one who works, his wage is not credited according to grace, but according to debt”? Why would he speak in terms of a wage (or a gift) from outside ourselves being credited to us by debt (or by grace)?
Would he not rather say something like, “Now to him who works, his works are credited as (= treated as) his righteousness according to debt (κατα όφείλημα, kata opheilēma)”? This would correspond nicely with verse 5 (“his faith is credited for righteousness”) if faith-credited-for-righteousness in fact means faith-treated-as-righteousness (which, I will try to show, it doesn’t). Thus Paul would accomplish what Gundry seems to think he wants: to show that our righteousness consists not of our works but does consist of our faith. But this is not the conceptual framework that Paul develops.
2. Piper cites several passages designed to show faith as the instrument to that by which we are reckoned righteous rather than that which is itself reckoned for righteousness (Romans 4:11, 10:10), and the points he makes in respect to each of these passages are worthy of consideration and more so when taken cumulatively, which is why I recommend the reader look at his analysis (pgs. 53-64 in particular in relation to this blog post). For the sake of brevity, however, I think that his best support for the general argument that Scripture teaches faith to be the instrument rather than the ground of justification is found in Philippians 3:9.
Another evidence that Paul does not intend for the phrase “faith is credited for righteousness” (4:5) to mean that our righteousness “consists of faith” (I, 8) is found in Philippians 3:8b-9.
I have suffered the loss of all things, and count them as rubbish in order that I might gain Christ, (9) and be found in him, not having a righteousness of my own from the law, but that which is through faith in Christ, the righteousness from God based on faith.
When Paul says that he aims to be found “in [Christ], not having a righteousness of my own,” does he mean that the righteousness he hopes to have in Christ is the righteousness that consists in his own faith? That is highly unlikely, because therighteousness that he aims to have is his by virtue of being “in Christ” (ἐν αὐτῷ, en autō) and is said to be “through faith” (διὰ πίστεως, dia pisteōs) and “based on faith” (ἐπὶ τῇ πίστει, epi tē pistei). The conceptual framework here is not that faith is our righteousness, but that, because of faith, we are united to Christ in whom we have a righteousness “from God” (τὴν ἐκ θεοῦ, tēn ek theou). This too supports our earlier conclusion that imputed righteousness is not “righteousness that consists in our faith,” but rather an external “righteousness credited to us because of our faith.”