Now we know that whatever the law says it speaks to those who are under the law, so that every mouth may be stopped, and the whole world may be held accountable to God.
19. Now we know, etc. Leaving the Gentiles, he distinctly addresses his words to the Jews; for he had a much more difficult work in subduing them, because they, though no less destitute of true righteousness than the Gentiles, yet covered themselves with the cloak of God’s covenant, as though it was a sufficient holiness to them to have been separated from the rest of...
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
For all have sinned,.... This is the general character of all mankind; all have sinned in Adam, are guilty by his sin, polluted with it, and condemned for it; all are sinners in themselves, and by their own actual transgressions; this is the case of the whole world, and of all the men in it; not only of the Gentiles, but of the Jews, and...