We can better appreciate today’s Gospel if we know the circumstances that brought Matthew to write it. We must go back to the total destruction of Jerusalem by the Romans in the year 70 a.d..
Not a stone was left upon a stone, and everyone in the city perished. However, the Pharisees, who had always cooperated with the Romans, were allowed to take away their families to settle in a campground they owned in a place called Jamnia on the Mediterranean coast.
They were left asking themselves how they could survive as a religion, when their religious life had centered on temple worship. The answer they came up with was that they would see the core of Judaism to consist in being strict observers of all the old and new mandates of their law.
They immediately saw that since their expanded view of the law forbade them to eat with Gentiles. Along with that, they saw that the tens of thousands of Jews who also saw themselves to be Christians would need to avoid mixing with Gentle Christians if they wanted to remain Jewish.
Looking at the situation retroactively, they began saying that Jesus, by mixing with Gentiles, even eating with them, had by his example set out to destroy the law and the Prophets.
In writing his Gospel to refute that claim, Matthew quoted Jesus as saying, “I did not come to destroy the law and the prophets. I came to fulfill them.”
In his Sermon on the Mount Jesus, one by one, took the mandates of the Old Law, showing how the Father had sent him to bring them to completeness. In God’ s eyes it is not enough to avoid adultery, we must also avoid lusting. It is not enough to love one’s neighbor, we must love all of God's children.