Then what becomes of our boasting? It is excluded. By what kind of law? By a law of works? No, but by the law of faith.
27. Where then is glorying? The Apostle, after having, with reasons abundantly strong, cast down men from their confidence in works, now triumphs over their folly: and this exulting conclusion was necessary; for on this subject, to teach us would not have been enough; it was necessary that the Holy Spirit should loudly thunder, in order to lay prostrate our loftiness.
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
Do we then make void the law through faith?.... Which question is answered by way of detestatation, God forbid! and by asserting the contrary, yea, we establish the law. The law is not made void, neither by the grace nor doctrine of faith: not by the grace of faith; for that faith is not right which is not attended with works of righteousness; and those...