What does Matthew 5:29 mean?
Jesus is expanding on His earlier comment, that righteousness worthy of heaven must be perfect (Matthew 5:20). The two examples given so far indicate that anger and lust are sins, just as much as murder and adultery (Matthew 5:21–22; 5:27–28).In this verse, Jesus uses a common technique. This is formally named hyperbole, or "exaggerating for effect." When a mother says, "I've told you a million times," or a manager says, "we're going to blow up our business model," those are examples of hyperbole. Those who hear or read those words understand them to be non-literal and meant to make a point about the situation at hand.
What's interesting about this statement is that Jesus is already overturning cultural assumptions. Saying lust is adultery of the heart, and anger is murder of the heart, would seem extreme and unsettling to His audience. At first, some might have wondered if Jesus really meant this as a literal, physical command. Quickly, though, it becomes obvious—especially in context with His other teaching—that this is not meant as a literal command.
However, Jesus is making an important point about how dangerous our thoughts and desires can be. Being led by our urges, rather than keeping our desires under control, is a path to destruction. In that vein, He says that it's worth losing an eye rather than letting that eye drag someone into sin, and by extension into hell. Jesus will make a similar comment about a person's hands (Matthew 5:30). The immediate context of this remark is the idea of lust: "looking" at someone with sexual intent.
Where Jesus is certainly not exaggerating is the idea that "looking with lustful intent" is enough to earn someone hell. God's will for His people is that they do not engage in lust. The kingdom of heaven requires righteousness that is perfect (Matthew 5:20), just as God is perfect. A major aspect of Jesus' gospel is that nobody can be righteous enough on their own to earn a place in the kingdom of heaven. His mission on earth included dying on the cross as the final blood sacrifice for sin. Without that covering and being declared righteous by God through faith in Christ, nobody will be saved (Romans 3:21–31).