Despite the decision of Acts 15:28-29, certain false teachers undermined the church, insisting that circumcision and other parts of the law were necessary. Paul called their teaching a different and damning gospel (Galatians 1:6-9). Some Christians were “bewitched” (Galatians 3:1) into accepting circumcision as a necessity. To them Paul said,
For freedom Christ has set us free; stand firm therefore, and do not submit again to a yoke of slavery. Look: I, Paul, say to you that if you accept circumcision, Christ will be of no advantage to you. I testify again to every man who accepts circumcision that he is obligated to keep the whole law. You are severed from Christ, you who would be justified by the law; you have fallen away from grace. (Galatians 5:1-4)
The issue was not circumcision as a social custom. These false teachers were making it, as a part of Moses’ law, a requirement. They wanted to be “justified”— made right with God—by law. As an inspired apostle, Paul fought against that return to the slavery of legalism. He was not against circumcision in itself. He was against leaders imposing circumcision, or anything like it, on Christians.