What does Matthew 5:21 mean?

In the preceding verse, Jesus set what seemed like an impossibly high standard. He declared that unless a person's righteousness exceeds or surpasses that of the scribes and Pharisees, he or she would never enter the kingdom of heaven (Matthew 5:20). Since the scribes and Pharisees were their culture's role models of extreme righteousness for many everyday Jewish people, how could anyone hope to get into the kingdom of heaven? Now Christ begins to expand on His meaning by discussing the connection between sins of action and sins of attitude.

In making His earlier statements, Jesus teaches two points, one immediate and part of a broader sense of what it means to be saved as a Christian. First, He implies that the scribes and Pharisees were not truly righteous enough to be allowed into the kingdom. Despite outward appearances, their supposed "perfection" did not reach their hearts. Second, and key to many of the illustrations in the Sermon on the Mount, is the idea that no person is worthy of heaven. This teaching will become essential to the gospel of Jesus.

In His first example, Jesus points to the teaching of the scribes and Pharisees by saying, "You have heard it said." He will use this phrase repeatedly to describe what was common knowledge about the Old Testament commands. Jesus does not necessarily mean that those statements are wrong, but He will often clarify how they should be interpreted. This command is clear enough. This is the sixth of the Ten Commandments: You shall not murder (Exodus 20:13). The judgment for murderers was also clear, prescribed as death (Numbers 35:31).

While "don't murder" seems like an easy standard to follow, Jesus is about to make a stunning connection between thoughts and actions. God's intent behind the sixth commandment goes far beyond "just" not killing people. It's meant to be applied at the deepest levels of our thoughts and feelings toward every single person.