"You have heard that it was said ... but I say to you." -- Jesus, on the ancient law of Israel; Sermon on the Mount.In the Sermon on the Mount, Jesus time and again starts with the ancient law of Israel, and goes beyond it. He calls us to righteousness that "surpasses that of the Scribes and the Pharisees", who kept the ancient law meticulously. "But their hearts are far from me," he says of us when we keep the letter of the law in cold hearts.
Some have looked at Jesus' comments on the ancient law and have said, "he is following the ancient Jewish tradition of putting a hedge around the law." In that strain of Jewish thought, if the Law said not to eat an animal cooked in the milk of its own mother, they would not even eat meat and dairy in the same meal to avoid any chance of breaking that law. A hedge would be built of laws of men forbidding things that might be innocent, and these innocent things were forbidden to keep people even further away from what the law opposed.
This, I think, is exactly what Jesus was not doing. "Their heart is far from me," he took up the prophet's refrain. "You have heard that it was said, 'You shall not kill', but I say to you that whosoever is angry with his brother without cause is in danger. You have heard that it was said, 'You shall not commit adultery', but I say to you that whoever looks on a woman with lust has already committed adultery with her in his heart." Here Jesus does not follow the human precept of creating a hedge around the law and forbidding innocent things. Instead, here he brings the law of God to bear even on our hearts, not forbidding innocent actions but forbidding guilty desires. He writes the commandments of God on our hearts.
Matthew shows Jesus summing up the entirety of the law twice: first as doing unto others what we would have them do unto us; next as loving God with all our heart, soul, and mind, and loving our neighbor as ourselves. The prophecy of Jeremiah shows that in the days of the Messiah, the law of the new covenant is to be written on our hearts. "Create in me a clean heart, O God" has always been the prayer of the faithful.