Adapted from The Reformation Study Bible.
The New Testament views Christian obedience as the practice of "good works." Christians are to be "rich in good works" (1 Tim. 6:18; cf. Matt. 5:16; Eph. 2:10; 2 Tim. 3:17; Titus 2:7, 14; 3:8, 14). A good deed is one done according to the right standard, God's revealed will; from a right motive, love for God and others; and with a right purpose, the glory of God.
Legalism is a distortion of obedience that can never produce good works in this sense. It skews motive and purpose, seeing good deeds as ways to earn God's favor. It can be arrogant and contemptuous of those who do not labor in the same way. Finally, legalism's self-advancing purpose squeezes humble kindness and compassion out of the heart.
Legalism is a distortion of obedience that can never produce good works...
Two Kinds of Legalism in the New Testament
In the New Testament we meet different kinds of legalism. Legalists among the Pharisees thought that because they were descended from Abraham they were guaranteed approval by God, while paradoxically they formalized daily observance of the law, down to minutest details, as the rule of life. In doing so they avoided what the law truly required. Judaizers were legalists who taught Christian believers that they must go on to become Jews by being circumcised and observing the religious calendar and ritual laws, and in this way gain favor with God. Jesus attacked the legalism of the Pharisees; Paul, the Judaizers.
The Pharisees that opposed Jesus thought of themselves as faithful keepers of the Mosaic law. Yet in emphasizing minor details they neglected what matters most (Matt. 23:23, 24). Their elaborate and misguided interpretations of the law denied its true spirit and aim (Matt. 15:3-9; 23:16-24). They substituted human tradition for God's authoritative law, binding consciences where God had left them free (Mark 2:16-3:6; 7:1-8). At heart they were hypocritical, seeking human approval for themselves and condemning others (Luke 20:45-47; Matt. 6:1-8; 23:2-7).
The Judaizers opposed by Paul added to the gospel requirements for salvation that obscured and denied the all-sufficiency of Christ (Gal. 3:1-3; 4:21; 5:2-6). The idea that there must be additional requirements to perfect the gospel was the root of their error. Paul opposed this idea no matter who advanced it (Col. 2:8-23), because it corrupted the way of salvation. Like Jesus, he would not tolerate those who brought new burdens to lay on the sheep.
Legalism erroneously teaches that there must be additional requirements to perfect the gospel.
Adapted from The Reformation Study Bible's theological article on "Legalism"