2 But grant that we have been in the wrong hitherto. What is to be done now? Shall we come under the law, since Christ has not destroyed, but fulfilled it? Shall we by circumcision add shame to shame, and believe that God is pleased with such sacraments? Shall we observe the rest of the Sabbath, and bind ourselves in the fetters of Saturn? Shall we glut the demon of the Jews, for he is not God, with the slaughter of bulls, rams, and goats, not to say of men; and adopt, only with greater cruelty, in obedience to the law and the prophets, the practices on account of which we abandoned idolatry?
Shall we, in fine, call the flesh of some animals clean, and that of others unclean, among which, according to the law and the prophets, swine's flesh has a particular defilement? Of course you will allow that as Christians we must not do any of these things, for you remember that Christ says that a man when circumcised becomes twofold a child of hell. It is plain also that Christ neither observed the Sabbath himself, nor commanded it to be observed. And regarding foods, he says expressly that man is not defiled by anything that goes into his mouth, but rather by the things which come out of it. Regarding sacrifices, too, he often says that God desires mercy, and not sacrifice. What becomes, then, of the statement that he came not to destroy the law, but to fulfill it?
If Christ said this, he must have meant something else, or, what is not to be thought of, he told a lie, or he never said it. No Christian will allow that Jesus spoke falsely; therefore he must either not have said this, or said it with another meaning.
Source: Reply to Faustus the Manichaean (New Advent)