But now the righteousness of God without law (i.e. apart from law) is (or, has been) manifested, being witnessed by the Law and the prophets. On the essential meaning of God's righteousness ( θεοῦ δικαιοσύνη), see on Romans 1:17, and Introduction.
This passage, in which the thesis of Romans 1:17 is formally enunciated, is consistent with this meaning; in confirmation of which observe Romans 1:25, Romans 1:26, where δικαιοσύνη αὐτοῦ evidently means God's own righteousness, as also above, Romans 1:5.
If this view is correct, there is no need to follow commentators into their discussions of the significance of χωρὶς νόμου in supposed connection with the idea of man's imputed righteousness; such as whether it is meant to declare justification through Christ to be without the aid of the Law—"sine legis adminiculo" (Calvin)—or to exclude all legal works, done before, or even after justification, from any share in the office of justification.
However true these positions may be, what is said here seems simply to mean that God's righteousness has been manifested in Christ in a different way, and on a different principle, from that of law. The principle of law is to enjoin and forbid, and to require complete obedience; but law, even as exhibited in the Divine Law of the Jews, has been shown to fail to enable man thus to attain to δικαιοσύνη; therefore, apart from this exacting principle, the righteousness of God is now revealed to man, embracing him in itself.
The absence of the article before νόμου here, and its insertion in the latter clause of the same verse, where the Mosaic Law is definitely referred to, is fully explained by what has been said above under Romans 2:13.
Being witnessed, etc., is introduced parenthetically by way of intimating that this manifestation of God's righteousness, though "apart from law," is not in any opposition to the teaching of the Law and the prophets, being, in fact, anticipated by them.
The proof of this appears afterwards in Romans 4:1-25.