Peter and Paul were preaching the gospel at the same time and sometimes in the same places. But, as the Encyclopedia of World Biography puts it, “there was a difference of opinion concerning doctrinal matters” between them. Others have said there was “deep seated conflict.” Basically, Biblical scholars try to politely say these two guys probably hated each other. And one thing they seriously clashed about was Christians having the snip.
Jewish men were famously circumcised, even back then. In the Old Testament, Abraham agrees to stick with God, and the sign of this special friendship is removing that loose skin. Since early followers of Jesus, like Peter, were Jews, they already had this operation. But once they started preaching to Gentiles, there was the question of whether followers of this new religion had to follow Jewish law. Sure, there were arguments about eating kosher, but most potential male converts cared more about the foreskin debate.
This was, after all, a time without sanitary surgery conditions and anesthesia. Undergoing circumcision would not only be painful but could literally kill you. But Peter was basically Jewish to the end. He didn’t think Jesus fundamentally changed anything but was the Jewish Messiah, and his followers needed to be Jewish in the genitals. Paul, leaning more Christian than Jewish, didn’t agree.
The pair eventually had to have a summit to resolve this. According to Galatians, they decided Peter would preach to circumcised people and Paul would minister to those who wanted to stay intact.