Six times in Matthew 5 Jesus introduces statements with, “But I say to you” (ESV). In each of these statements in the Sermon on the Mount, Jesus claims to have a higher authority than the scribes. The Lord, as the original giver of the law, was expounding on the law and bringing out its full meaning.
Jesus first says, “But I say to you” after quoting the Mosaic Law’s prohibition of murder (Matthew 5:21). Jesus then contrasts the well-known saying that “you shall not commit murder” (Exodus 20:13) with an explanation that a person didn’t have to physically commit murder to be guilty of murder. A person is guilty of murder in his heart even if he were only angry with his brother (Matthew 5:22). Unrighteous anger makes one guilty and worthy of hell. Other teachers of the law taught that, as long as you didn’t literally commit murder, you were okay; Jesus said, no, you’re not off the hook that easily, because God sees the heart.
Jesus then recounts the Mosaic requirement to not commit adultery (Matthew 5:27; cf. Exodus 20:14). He adds, “But I say to you” that, if a man looks at a woman in lust, that man is guilty of adultery in his heart (Matthew 5:28). Jesus then reminds His listeners of the law permitting divorce (Matthew 5:31) and then pivots: “But I say to you” that, if a person divorces his wife for any reason other than immorality, he causes her to commit adultery and commits adultery himself if he remarries someone else (Matthew 5:32).
Jesus cites the law prohibiting making false vows (Matthew 5:33; cf. Leviticus 19:12). He then adds, “But I say to you, Do not take an oath at all” (Matthew 5:34, ESV). Jesus notes the punitive standard of an eye for an eye and a tooth for a tooth (Matthew 5:38; cf. Exodus 21:24) and adds, “But I say to you,” that one should not resist an evil person but be generous to him (Matthew 5:39). Finally, Jesus reminds His listeners of their responsibility to love their neighbors (Matthew 5:43) and challenges them to go further: “But I say to you, love your enemies and pray for those who persecute you” (Matthew 5:44).
In each of these cases, Jesus is challenging His listeners’ perception of how a person becomes righteous. Many thought that they could simply be externally obedient to the Mosaic Law, and, if they did that, they would be righteous in God’s sight. Earlier, Jesus had instructed the crowds that they needed to repent or change their mind about how they could become righteous and how they could enter the kingdom, and they needed to do it quickly because the kingdom was near (Matthew 4:17). External obedience doesn’t equate to righteousness. In the Sermon on the Mount, Jesus explained that they needed to have an internal righteousness—a kind of righteousness neither they nor the Pharisees had (Matthew 5:20). As Jesus would later explain, righteousness, eternal life, and entrance into the kingdom come through belief in Him (John 3:16; 6:47) and not from obedience to the law.
Jesus’ “but I say to you” statements contrast the people’s understanding of the legal requirements for righteousness by obedience to the law and the people’s actual need for righteousness by belief in Jesus. Before Jesus offers them the solution (Himself as the water of life, the bread of life, etc.), He shows them their need. While many people did believe in Him for eternal life, the leadership of the nation refused to change their minds about how they could have righteousness, choosing to depend on their own efforts rather than putting their trust in their Messiah. In so doing, they missed the kingdom (see Matthew 21:31, 43).