Legalism is strict adherence to a literal interpretation of law. If the law is valid and applicable then legalism is appropriate and virtuous. If the law is either invalid or irrelevant then legalism is a mistake and will lead a person astray. In Christianity legalism is a problem that results from both invalid laws and valid but irrelevant laws.
Let’s start with valid but irrelevant laws. When God made a covenant with Moses and Israel He gave them “The Law”. That is, God gave them a code of conduct that revealed His will for them. If they “kept the law” then He would bless them. When God sent Jesus to earth He gave humans a new way to please Him and gain His blessing. As Paul says, the law of Moses was replaced with the law of the Spirit. Therefore, to “keep the law” today is not going to work as a means of pleasing God. In theological terms we say that “justification is by faith and not by the works of the law”. An attempt to be made right with God by legalism is therefore a bad idea.
Secondly, Christians often attempt to please God by following their own “law”. That is, many Christians will invent a whole series of cultural and moral codes (don’t drink, don’t gamble, don’t smoke, don’t swear, don’t …………) and teach that following these rules will make them good Christians that are pleasing to God. This too is a bad idea.
So what is the attraction of legalism? The truth is, legalism is easier than true spirituality. To follow the law of Christ and to walk in the Spirit requires a real relationship with God. It is a lot easier to follow rules than to follow Jesus. Following the Law depends on using will power alone. Following Jesus takes us beyond will power alone into receiving help from God in aiding our will. However, receiving help from God requires a relationship with Him and that requires humility and honesty with Him.
Furthermore, confusion arises because we all sense intuitively that God does want us to obey Him, and that “The Law” does reveal His will. Christians who attempt to accept “justification by faith” and then neglect moral issues do not find peace with God. Paul gives us the answer to this problem in Romans chapter 8, where he teaches that “what the law could not do since it was weakened by the flesh, God does in us as we follow the Spirit”. Christians do end up using volition as they follow Jesus, because they have to use their will to seek Him. Humans also have a role in their sanctification which also is partly accomplish by using will power. Choosing to “follow Jesus” involves difficult and painful activities like resisting the devil and crucifying the flesh. Mostly though Christian effort is needed to admit that you need help to become fully devoted and victorious in a “spiritual” life. It is attempting to follow Jesus without really knowing Him that causes problems. That is, we are not called to merely following Jesus teachings, but to actually know Him, trust Him, depend on Him and follow Him through a relationship with Him. This is much more difficult than legalism.
See also Obedience.