In Acts 15 Paul makes the point that circumsision isn’t required for salvation and asks why they are putting a burden on new believers that they themselves couldn’t live up to (fulfilling the law). Then in chapter 16 he circumcised Timothy. On first glance it seems inconsistent. But the nuts and bolts of the issue is that in chapter 15 the Jews were making it compulsory and claiming that in order to be saved the gentiles had to be circumcised for salvation Acts 15:1. In verse 16 it was done for the sake of ministry. If Timothy hadn’t been circumcised it would have been more difficult to witness to Jews. Circumcising Timothy was part of Paul’s desire to be all things to all people 1Cor 9:19-23.