Albert Barnes on Acts 18:13

Contrary to the law. Evidently intending contrary to all law--the laws of the Romans and of the Jews. It was permitted to the Jews to worship God according to their own views in Greece; but they could easily pretend that Paul had departed from that mode of worshipping God. It was easy for them to maintain that he taught contrary to the laws of the Romans, and their acknowledged religion; and their design seems to have been, to accuse him of teaching men to worship God in an unlawful and irregular way, a way unknown to any of the laws of the empire.