Earlier in the Sermon on the Mount, Jesus makes a claim that should have emptied the synagogue: unless your righteousness exceeds that of the scribes and Pharisees, you will never enter the kingdom of heaven. The scribes and Pharisees were the religious overachievers of their day. They knew the text. They kept the rules. And Jesus says their righteousness is not enough.
Why? Because it was performance, not obedience. Keener notes that the Pharisaic tradition had developed an elaborate fence of practices around the law, a system for keeping the commandments so scrupulously that a sincere man could convince himself he had arrived. He had arrived at what? Not at love for God, but at successful performance for God. Adams, writing for counselors, gives the diagnosis in one line: it is exhausting to perform, and it is empty.
You can keep the outward form of the commandment while rotting on the inside. You can smile through a Sunday while your heart is dark. The Pharisees had never actually kept the commandment; they had only acted like they had. Performance and obedience look similar from the outside. They are nothing alike on the inside.
Paul, himself a former Pharisee, tells us what rescued him in Philippians 3. He counted his own performance as loss for the sake of Christ, that he might be found in him, not having a righteousness of his own that comes from the law, but the one that comes through faith in Christ. That is the only righteousness that exceeds the scribes and Pharisees. It is not yours. It is Christ’s, received by faith, counted to you as gift.