In light of the gospel, let me especially demolish the myth that legalism is a blunder that's associated only with our initial salvation—with our positional justification in God's eyes. Most believers realize we could never earn such salvation; we've come to accept that no one can work his way into God's kingdom… .
But when it comes to our sanctification, suddenly we become legalists. In the matter of maturing in Christlikeness—and in continuing to please God and find favor with God and acceptance with God—we suppose it's all about what we have to accomplish ourselves and all the rules and standards and values we need to adhere to. We seem to inherently assume that our performance is what will finally determine whether our relationship with God is good or bad: so much good behavior from us generates so much affection from God, or so much bad behavior from us generates so much anger from God.
We get the Christian life all backwards. It subtly becomes all about us and what we do (which leads to slavery) instead of being all about Jesus and what he's done (which leads to freedom). We may not articulate all this theologically, but it sure comes out in the way we live.
By their behavior, legalists essentially are saying this: "I live the Christian life by the rules—rules that I establish for myself as well as those I expect others to abide by." They develop specific requirements of behavior beyond what the Bible teaches, and they make observance of those requirements the means by which they judge the acceptability of others in the church.
We've all become pretty adept at establishing these rules and standards that we find personally achievable. Legalism ...