3 As "the law and the prophets" may have three different meanings, it is uncertain in what sense the words are used by Jesus, though we may form a conjecture from what follows. For if Jesus had gone on to speak of circumcision, and Sabbaths, and sacrifices, and the observances of the Hebrews, and had added something as a fulfillment, there could have been no doubt that it was the law and the prophets of the Jews of which He said that He came not to destroy, but to fulfill them.
But Christ, without any allusion to these, speaks only of commandments which date from the earliest times: "You shall not kill; You shall not commit adultery; You shall not bear false witness." These, it can be proved, were of old promulgated in the world by Enoch and Seth, and the other righteous men, to whom the precepts were delivered by angels of lofty rank, in order to tame the savage nature of men. From this it appears that Jesus spoke of the law and the prophets of truth.
And so we find him giving a fulfillment of those precepts already quoted. "You have heard," He says, "that it was said by them of old time, You shall not kill; but I say unto you, Be not even angry." This is the fulfillment. Again: "You have heard that it was said, You shall not commit adultery; but I say unto you, Do not lust even." This is the fulfillment. Again: "It has been said, You shall not bear false witness; but I say unto you, Swear not." This too is the fulfillment.
He thus both confirms the old precepts and supplies their defects. Where He seems to speak of some Jewish precepts, instead of fulfilling them, He substitutes for them precepts of an opposite tendency. He proceeds thus: "You have heard that it has been said, An eye for an eye, and a tooth for a tooth; but I say unto you, Whosoever shall smite you on your right cheek, turn to him the other also." This is not fulfillment, but destruction. Again: "It has been said, You shall love your friend, and hate your enemy; but I say unto you, Love your enemies, and pray for your persecutors."
This too is destruction. Again: "It has been said, Whosoever shall put away his wife, let him give her a writing of divorcement; but I say unto you, That whosoever shall put away his wife, saving for the cause of fornication, causes her to commit adultery, and is himself an adulterer if he afterwards marries another woman." These precepts are evidently destroyed because they are the precepts of Moses; while the others are fulfilled because they are the precepts of the righteous men of antiquity.
If you agree to this explanation, we may allow that Jesus said that he came not to destroy the law, but to fulfill it. If you disapprove of this explanation, give one of your own. Only beware of making Jesus a liar, and of making yourself a Jew, by binding yourself to fulfill the law because Christ did not destroy it.
Source: Reply to Faustus the Manichaean (New Advent)