In his essay “The Theme of Romans” (from Karl P. Donfried, ed., The Romans Debate, 2nd Ed., 338) Peter Stuhlmacher makes this important observation that should factor into discussions on the oft debate phrase δικαιοσύνη θεοῦ (e.g. Rom. 1.17):

Even the expression “the righteousness of God,” which is so difficult for us to understand, need not have been unknown to those whom Paul addressed. Again and again in the Psalms, in (Deutero-) Isaiah, and in Israelite prayers of repentance God’s salvific righteousness is mentioned, and the concept of the “righteousness of God” is also used in the Gospels (cf. Matt. 6.33) and in James (cf. Jas. 1.20). This famous concept was in no way unintelligible. The question was only how Paul wanted to nuance it.

Stuhlmacher emphasizes that many of Paul’s early converts were from synagogues. They would have been familiar with Torah and the language of Torah. Therefore, as we wrestle with obscure sayings from Paul, we should ask if the concept can be found anywhere in Scripture.

Stuhlmacher doesn’t deny that we must discover how Paul nuanced the term “righteousness of God”, but neither should we feel like he made the statement in a vacuum.