But now the righteousness of God has been manifested apart from the law, although the Law and the Prophets bear witness to it—
21. But now without the law, etc. It is not certain for what distinct reason he calls that the righteousness of God, which we obtain by faith; whether it be, because it can alone stand before God, or because the Lord in his mercy confers it on us. As both interpretations are suitable, we contend for neither.
From all this Paul infers that it is in vain to look for justification by the works of the law, and that it is to be had only by faith, which is the point he has been all along proving, from Rom 1:17, and which he lays down (Rom 3:28) as the summary of his discourse, with a quod erat demonstrandum - which was to be demonstrated.
Commenting on Romans 3:19-31
Whom God had set forth to be a propitiation,.... Redemption by Christ is here further explained, by his being "a propitiation": which word may design either Christ the propitiator, the author of peace and reconciliation; or the propitiatory sacrifice, by which he is so; and both in allusion to the mercy seat, which was a type of him as such.