These things are true, because this is the task of the law: to teach the ignorant, to subject the wicked to God, to provoke those who by the worship of idols are ungodly to trust in a better hope by the promise which is given through the law. The teacher of the law is right to glory in these things, because he is teaching the form of truth. But if the teacher does not accept the Expected One whom the law has promised, he glories in vain in the law, to which he is doing harm as long as he rejects the Christ who is promised in the law. In that case he is no more learned than the fools, nor is he a teacher of children, nor is he a light to those who are in the darkness, but rather he is leading all of these into perdition.
Here again he says not, in the conscience and in actions and in well-doings, but "in the Law;" and after saying so, he does here also what he did with regard to the Gentiles. For as there he says, "for wherein you judge another, you condemn yourself," so says he here also.
Paul says that they had knowledge and truth not in their conscience, their actions or their well-doing but in the law. After saying this he repeats what he has already said to the Gentiles: In judging others they condemn themselves.
[AD 384] Ambrosiaster on Romans 2:20