Today, in honoring St. Matthew, we should recall the causes he had for writing his Gospel. It began with the Romans destroying Jerusalem and its temple in the year 70 A.D.
Back then, there had been a sizable remnant of the Pharisees that had established itself at a town called Jamnia. There, they found themselves struggling with an identity crisis. They had always seen themselves as the People of the Temple, but without the Temple, who were they?
As a conclusion to their problem, they came to decide that to be a Jew, one had to avoid eating with Gentiles, and had to observe the many dietary rules laid down since the time of Uriah and Nehemiah, six centuries before.
With that, the Pharisees decreed that those who had become Christians could no longer be considered true Israelites, Further, they announced that Jesus had come to destroy the Law and the Prophets.
In response to those accusations, Matthew demonstrated how Jesus, for from destroying the Law and Prophets, showed by his Sermon on the Mount ,that he had come to fulfill them.