What does Galatians 5:3 mean?

Paul often repeats things to stress how important the point is. Here he repeats what he said in the previous verse and adds to it. The Galatians were being pressured by the Judaizers to get circumcised in order to be welcomed into the family of God (Galatians 2:4). Paul has urged them to believe that they have already been fully welcomed into God's family as His very children. Circumcision can do nothing to add to what Christ has already done for them through their faith in him (Galatians 3:25–29).

Some of these Galatian men might have been tempted to go along with being circumcised, though, just to cover all their bases. After all, what could it hurt, spiritually speaking?

Paul is not saying that circumcision, in and of itself, is wrong. What he is saying is this: the Galatians can't trust in Christ alone for their salvation and also be circumcised "just in case." To add something to faith in Christ in the hope of being fully accepted by God is to give up on Christ completely. Any addition of works, or rituals, or sacraments is a complete rejection of a gospel of grace through faith (Romans 11:6).

Paul says that anyone who chooses circumcision to be accepted by God is actually signing up to follow the entire law of Moses. That person is asking God to judge him according to his works and not according to Christ's works and death in his place on the cross. This suggests that such a person does not trust Christ for their salvation.