In the Sermon on the Mount, where Jesus reveals to us what life is like in His Kingdom, He contrasts the Old Testament Law with its true intent. And it does this by saying, "You have heard that it was said to those of old... but I say unto you." Or, to put it another way, "You have an understanding about the Law and what it governs, but I want to show you the true intent of the Law and what it really means."
The Law governed external actions. Or so it seemed to them and to us. But in the Sermon on the Mount Jesus shows us the true intent of the Law by contrasting it to the human understanding of it. In other words, only actions matter in the mind of men. But with God, everything comes from the heart.
"For the LORD does not see as man sees; for man looks at the outward appearance, but the LORD looks at the heart” (1 Samuel 16:7).
Do you want to know more about having a heart that is pleasing to the Lord? Good. Then keep listening.
The following is a study on Matthew 5:21.