In the Sermon on the Mount, Jesus says several times, “You have heard that it was said . . .” and follows it up with “But I tell you. . . .” Using the formula, Jesus creates a contrast between the Law of Moses (as interpreted by the Pharisees and scribes) and His own command. In doing this, Jesus is obviously claiming an authority greater than that of the scribes. But it also seems like He is negating the Law.
Rest assured, Jesus did not contradict the Law in any point. In the same sermon, Jesus makes sure no one misunderstood: “Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them” (Matthew 5:17).
Jesus’ point in the Sermon on the Mount was that God sees the heart, and that we are actually held to a higher standard than external conformity to a set of rules. The Pharisees taught that, as long as you did the right things, you were “holy.” Jesus said not so; “For I tell you that unless your righteousness surpasses that of the Pharisees and the teachers of the law, you will certainly not enter the kingdom of heaven” (Matthew 5:20).
Jesus is teaching the necessity of having a heart to follow God. Putting on an act and going through the motions of serving God is dishonest. It is also futile, because God sees through the masks we wear (see Mark 7:6). A person who pretends to be holy on the outside yet nurtures a sinful heart is a hypocrite. The Pharisees, who everyone thought were holy, were guilty of just such dissimulation. God doesn’t want more religious activity; He wants a heart dedicated to Him. Holiness starts on the inside.
In Matthew 5:21-22, for example, Jesus says, “You have heard that it was said to the people long ago, ‘You shall not murder, and anyone who murders will be subject to judgment.’ But I tell you that anyone who is angry with a brother or sister will be subject to judgment.” The external command was “do not murder.” This is a good command; we should not murder people. But we make a mistake if we think that’s where our responsibility ends. Jesus said, in essence, God sees your heart. If you have hatred in your heart, then you are just as guilty as the murderer in God’s eyes. The Pharisees’ attitude was, “I am good; I haven’t murdered anyone.” Jesus countered, “No, you are guilty because there is murder in your heart.”
It’s the difference between the letter of the Law and the spirit behind it. Keeping the letter of the Law doesn’t make you righteous (see Galatians 2:16). None of us can keep the whole Law perfectly, anyway. God requires a heart transformation; we must be born again (John 3:7).
God is looking for more than the external practice of religion. People might seem holy to other people, but that’s not the standard. Over and over, the Bible stresses purity of heart before the One who examines the heart (1 Chronicles 29:17; Psalm 24:4; 51:10; Matthew 5:8; 2 Timothy 2:22).
The Law was good and proper. Its purpose was to define sin and delineate holiness. Jesus was not in any way negating the Law; He was showing us the reason for the Law. Jesus fulfilled the Law in that He was the only person to ever keep the whole Law, even in His heart, without sin (Hebrews 4:15).