2 The passage itself, in which Christ tells the Jews not to think that He came to destroy the law, is rather designed to show that He did destroy it. For, had He not done something of the kind, the Jews would not have suspected Him. His words are: "Think not that I have come to destroy the law." Suppose the Jews had replied, What actions of yours might lead us to suspect this? Is it because you expose circumcision, breakest the Sabbath, discardest sacrifices, makest no distinction in foods?
This would be the natural answer to the words, Think not. The Jews had the best possible reason for thinking that Jesus destroyed the law. If this was not to destroy the law, what is? But, indeed, the law and the prophets consider themselves already so faultlessly perfect, that they have no desire to be fulfilled. Their author and father condemns adding to them as much as taking away anything from them; as we read in Deuteronomy: "These precepts which I deliver unto you this day, O Israel, you shall observe to do; you shall not turn aside from them to the right hand or to the left; you shall not add thereto nor diminish from it, that your God may bless you." Whether, therefore, Jesus turned aside to the right by adding to the law and the prophets in order to fulfill them, or to the left in taking away from them to destroy them, either way he offended the author of the law.
So this verse must either have some other meaning, or be spurious.
Source: Reply to Faustus the Manichaean (New Advent)