1. Legalism kills joyful Christian living. Paul asks the Galatians who have moved away from him and the gospel of the grace of God "What then has become of the blessing you felt?" (Gal. 4:15). There is a clear connection between the truth of justification by faith alone and healthy and vibrant Christian experience.
2. Legalism kills Christian fellowship. Legalism erects a barrier between people. That's what happened in Galatians 2:11-14 when the obedience and blood of Christ as the ground of justification was rejected as the basis of fellowship.
I once heard the story of how Christians in the Congo would sometimes greet each other by asking “Fresh milk?” (by which they meant "when did you last have your quiet time?"). And you would have to say when that was, and what it was that you had read. In itself that is not a bad thing, but there is only a short a gap between that and legalism (is it not a "how are you performing?" kind of question) . It is not hard to imagine the Pharisees asking each other that question. Perhaps those Christians should have been asking "are you repenting and trusting in Christ alone to save you?"
And of course the reason why legalism kills joyful Christian living, and kills Christian fellowship, is because:
3. Legalism kills the gospel.
The gospel is transformed from being the good news of our acceptance in Christ through faith alone, to one of our acceptance by God by relying on the works of the law. Paul emphasises this throughout the letter. Take a look at 1:6-7; and 2:15-16, 21; 3:10-11, 21; 4:21; 5:3-4.
So what is it? What is legalism?
C. J. Mahaney has a helpful definition:
Legalism is seeking to achieve forgiveness from God and acceptance by God through obedience to God.And this longer explanation from Dominic Smart is very perceptive:
It’s a way of making and keeping yourself acceptable to God. From this flows the legalism that is directed towards one another. It’s a way of scoring sanctity points in church. The need for order, structures and boundaries feeds our quest for control.
Our very ability to keep some rules feeds our pride and gives us the impression that our relationship with God is somehow founded upon this ability. It often arises out of a good motive: to be holy. It takes our faith away from Christ's sufficiency and misplaces it upon ours. We live to achieve his approval; we forget that we are already alive and accepted in Christ.
It is like the conversation that goes on every week in clothes shops all across the land. The wife tries on an outfit and says to her husband “how do I look?” and a pre-recorded message says “you look great” (the eyes are open, the mouth moves, but Mr Brain has long since been thinking about sport). Legalism makes us ask “do I look good?” and the answer we want is “yes, you look good." Legalism sets us out on the treadmill of performance.
Don't confuse legalism with obeying God's commands, or a concern to obey God's commands, the two are not the same.
Consider what Jesus says in John 15:10, 12, 14:
“If you obey my commands, you will remain in my love, just as I have obeyed my Father's commands and remain in his love. My command is this: Love each other as I have loved you. You are my friends if you do what I command.”
Legalism is not the desire to obey God's commands, it is the desire to keep God's commands with the wrong motives (to earn acceptance).