Legalism breeds a sense of entitlement that turns us into complainers.

Legalism says God will love us if we change. The gospel says God will change us because He loves us.

one primary enemy of the Gospel—legalism—comes in two forms. Some people avoid the gospel and try to save themselves by keeping the rules, doing what they’re told, maintaining the standards, and so on (you could call this “front-door legalism”). Other people avoid the gospel and try to save themselves by breaking the rules, doing whatever they want, developing their own autonomous standards, and so on (you could call this “back-door legalism”).

We make a big mistake when we conclude that the law is the answer to bad behavior. In fact, the law alone stirs up more of such behavior. People get worse, not better, when you lay down the law. To be sure, the Spirit does use both God's law and God's gospel in our sanctification. But the law and the gospel do very different things.

The ironic thing about legalism is that it not only doesn't make people work harder, it makes them give up. Moralism doesn't produce morality; rather, it produces immorality.