5 Every one can see the weakness of the argument that Christ could not have said, "Think not that I have come to destroy the law and the prophets: I came not to destroy, but to fulfill," unless He had done something to create a suspicion of this kind. Of course, we grant that the unenlightened Jews may have looked upon Christ as the destroyer of the law and the prophets; but their very suspicion makes it certain that the true and truthful One, in saying that He came not to destroy the law and the prophets, referred to no other law than that of the Jews.
This is proved by the words that follow: "Verily, verily, I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law till all be fulfilled. Whosoever therefore shall break one of the least of these commandments, and shall teach men so, shall be called the least in the kingdom of heaven. But whosoever shall do and teach them, shall be called great in the kingdom of heaven." This applied to the Pharisees, who taught the law in word, while they broke it in deed.
Christ says of the Pharisees in another place, "What they say, that do; but do not after their works: for they say, and do not." So here also He adds, "For I say unto you, Unless your righteousness exceeds the righteousness of the scribes and Pharisees, you shall not enter into the kingdom of heaven;" that is, Unless you shall both do and teach what they teach without doing, you shall not enter into the kingdom of heaven. This law, therefore, which the Pharisees taught without keeping it, Christ says He came not to destroy, but to fulfill; for this was the law connected with the seat of Moses in which the Pharisees sat, who because they said without doing, are to be heard, but not to be imitated.
Source: Reply to Faustus the Manichaean (New Advent)