Adam Clarke on Romans 3:19

What things soever the law saith - That the word law, here, does not mean the pentateuch, is evident from the preceding quotations, not one of which is taken from that work. Either the term law must here mean the Jewish writings in general, or that rule of moral conduct which God had given to both Jews and Gentiles: to the former in their own Scriptures; to the latter in that law written in their hearts by his own Spirit, and acknowledged in their written codes, and in their pleadings in every civil case. Now, according to this great law, this rule of moral conduct, whether given in a written revelation, as to the Jews, or by the secret inspiration of his Spirit, as in certain cases to the Gentiles, every mouth must be stopped, and the whole world, πας ο κοσμος, both Jews and Gentiles, stand convicted before God: for all mankind have sinned against this law.