Church Fathers on Matthew 5:19

Having now exhorted His hearers to undergo all things for righteousness' sake, and also not to hide what they should receive, but to learn more for others' sake, that they may teach others, He now goes on to tell them what they should teach, as though He had been asked, 'What is this which you would not have hid, and for which you would have all things endured? Are you about to speak any thing beyond what is written in the Law and the Prophets;' hence it is He says, 'Think…

And that for two reasons. First, that by these words He might admonish His disciples, that as He fulfilled the Law, so they should strive to fulfil it. Secondly, because the Jews would falsely accuse them as subverting the Law, therefore he answers the calumny beforehand, but in such a manner as that He should not be thought to come simply to preach the Law as the Prophets had done.

He here asserts two things; He denies that He was come to subvert the Law, and affirms that He was come to fulfil it.