Here’s the mistake the legalist makes. He confuses his own ongoing participation in the process of sanctification with God’s finished work in justification. In other words, he thinks that godly practices and good works somehow contribute to his justification. But God’s Word is clear when it says, “Therefore no one will be declared righteous in his sight by observing the law…” (Romans 3:20). None of us earn God’s approval and love by our good works. None of us can add to the finished, complete work of Jesus on the cross. He paid the price of our sins. He satisfied God’s wrath… The legalist allows his performance of spiritual duties to become his preoccupation and source of self-righteous pride. In doing so, he unwittingly walks away from the main thing – the gospel.
C.J. Mahaney