For there are many who are insubordinate, empty talkers and deceivers, especially those of the circumcision party.
Titus 1:10

Throughout his career, Paul battled a group of Jewish believers who taught that new converts must be circumcised and undergo ritual conversion to Judaism in order to be saved. Unfortunately, those people are still around today in every Christian and Jewish denomination, even if the specific man-made rules they preach have changed.