Ye have heard that it was said by them of old time, Thou shall kill; and whosoever shall kill shall be in danger of the judgment: But I say unto you, That whosoever is angry with His brother without a cause shall be in danger of the judgment. Matt. 5:21, 22.
With Matthew 5:21 we have come to the first of six illustrations of how our righteousness must exceed that of the scribes and Pharisees (verse 20). In those six examples we find Jesus filling up the meaning of the law (verse 17).
Common to each illustration are the words "you have heard that it was said" and "I say unto you." Those who had done the initial saying were such Jewish leaders as the scribes and Pharisees, who had taken God's Old Testament law and created oral tradition to protect that law and to apply it to the life of the people. Such Jewish leaders were generally sincere in their attempts to make the law meaningful. But their sincerity did not protect them from error.
That is why Jesus comes in with His "but I say." Those words are of crucial importance in understanding both Matthew 5:21-48 and the entire Sermon on the Mount. In them Jesus is pronouncing Himself to be the authority on the meaning of the law. But He was not basing His teaching on the judgment of others. To the contrary, He was the authority on the law. He approaches it not as a mere teacher, but as the lawgiver, the One who knows the height and depth of the law because He is the God who gave it in the first place.
In the process, He overturns pharisaic approaches to the law. The Pharisees, as Jesus will demonstrate, were excellent on the letter of the law, but poor in its spirit. They were perfectionists at heart, and all perfectionists need a list of do's and don'ts. To make the law manageable if they are going to obey it perfectly, they need to cut it down to size. Pharisees of all ages have done that by being careful in how they define sin. For them sin tended to be actions.
But Jesus overturns all such attempts. Refusing to play the pharisaic game, He moved beyond the outward letter of the law to its inward spiritual intent. Thus He pointed out that the root problem is not the act but the thought and attitude behind it. In that way Jesus shattered the easy perfectionism of the Pharisees of both His day and ours.
Lord in heaven, help me to grasp in my heart the full meaning of the law and not merely its outward shell.