Sam Storms, Tough Topics: Biblical Answers to 25 Challenging Questions (Wheaton: Crossway, 2013), 311–12, 314–15 [32-page sample PDF]:
Legalism has been defined in a number of ways, but here is my attempt: Legalism is the tendency to regard as divine law things that God has neither required nor forbidden in Scripture, and the corresponding inclination to look with suspicion on others for their failure or refusal to conform. . . .
2. Do you elevate to the status of moral law something the Bible does not require? . . .
Hold your conviction with passion and zeal, but do not seek to enslave the consciences of others who may disagree with you. . . .
One unmistakable sign of a legalistic spirit is the tendency always to be looking for what’s wrong in other people’s lives in order to judge them, instead of looking for what’s right in order to encourage them. . . .
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.
Here’s an antidote.