2 Supposing, then, that we agree in condemning the writers, we may succeed in vindicating the law and the prophets. By the law must be understood not circumcision, or Sabbaths, or sacrifices, or the other Jewish observances, but the true law, viz., You shall not kill, You shall not commit adultery, You shall not bear false witness, and so on. To this law, promulgated throughout the world, that is, at the commencement of the present constitution of the world, the Hebrew writers did violence, by infecting it with the pollution of their disgusting precepts about circumcision and sacrifice.
As a friend of the law, you should join with me in condemning the Jews for injuring the law by this mixture of unsuitable precepts. Plainly, you must be aware that these precepts are not the law, or any part of the law, since you claim to be righteous, though you make no attempt to keep the precepts. In seeking to lead a righteous life, you pay great regard to the commandments which forbid sinful actions, while you take no notice of the Jewish observances; which would be unjustifiable if they were one and the same law.
You resent as a foul reproach being called negligent of the precept, "You shall not kill," or "You shall not commit adultery." And if you showed the same resentment at being called uncircumcised, or negligent of the Sabbath, it would be evident that you considered both to be the law and the commandment of God. In fact, however, you consider the honor and glory of keeping the one no way endangered by disregard of the other. It is plain, as I have said, that these observances are not the law, but a disfigurement of the law.
If we condemn them, it is not as being genuine, but as spurious. In this condemnation there is no reproach of the law, or of God its author, but only of those who published their shocking superstitions under these names. If we sometimes abuse the venerable name of law in attacking the Jewish precepts, the fault is yours, for refusing to distinguish between Hebrew observances and the law. Only restore to the law its proper dignity, by removing these foul Israelitish blots; grant that these writers are guilty of disfiguring the law, and you will see at once that we are the enemies not of the law, but of Judaism. You are misled by the word law; for you do not know to what that name properly belongs.
Source: Reply to Faustus the Manichaean (New Advent)