Legalism among Christian disciples is the product of misplaced or misundertood trust that diminishes grace.
From Sam Storms:
Legalists feel good when they can identify another person’s errors. It reinforces their feelings of superiority. They actually think themselves more spiritual, more godly, and more favored and loved by God.
There’s a flip side to the legalistic spirit. In addition to being quick and dogmatic in identifying the small and rare failures of others, the legalist never acknowledges his own faults and failures. To admit and confess to sin or misjudgment is to run the risk of losing power, losing face, or losing prestige.
What drives this spirit? It is the belief that one’s own efforts and achievements merit acceptance with God and approval from men. Instead of resting in Christ’s achievements, confident of what he has done for us, legalists redouble their own works and take pride in what they do in view of what others don’t.
Look again at Mark 2:24: “And the Pharisees were saying to him, ‘Look, why are they doing what is not lawful on the Sabbath?’” Or again, Mark 3:2: “they watched him closely” (niv). That’s the legalists’ spirit: always on the lookout for someone else’s sin; always scanning the horizon for someone’s failure to measure up to their rules, rules that aren’t in the Bible; always spying on the behavior and beliefs of the other person to root out the slightest deviation from their traditions. They nitpick and judge, nitpick and judge, nitpick and judge!
Read the whole post at the Crossway Blog.