The Apostle Paul faced a severe dilemma in the early days of the Church; circumcision, commanded by God (Ge 17:10) as an expression of saving faith (Ro 4:11), and obediently observed by Abraham and the patriarchs, had also come to represent ritual conversion to Judaism (Ac 15:1), a religion teaching legalism: salvation by works (Ga 5:4), entirely contrary to justification by faith. (2-3)
Should Paul discourage obedience to one of God’s core commands (Mt 5:19) now that it’s been twisted into the foremost expression of rejecting God’s salvation? symbolic of earning salvation by works? (Ro 10:3) How could he neglect a plain command of God in good conscience, knowing saving faith establishes the law? (Ro 3:31) Yet how could he encourage obedience here without compromising the gospel? (1Co 9:22-23)
Paul circumcised Timothy (Ac 16:3), evidently not as a convert to Judaism, but to fulfill Torah as a good testimony to the Jews in his community, since Timothy would be a constant, faithful fellow worker with Paul throughout his ministry. (1Co 4:17)
However, when Titus was being pressured into ritual conversion to Judaism Paul objected fiercely, understanding this as a direct denial and corruption of the gospel. (Ga 2:3-5) Making a severe and costly break with the legalistic traditions of his people (5:11), Paul concluded those pushing Judaism on the Gentile saints as a condition of salvation were unsaved and cursed (1:10); he even wanted God to kill them. (5:12)
Paul clearly taught that those who converted to and depended on Judaism for salvation were not trusting Christ and were unsaved. (Ga 5:2-3) However, though he was accused of teaching the Jews to forsake circumcision (Ac 21:21), both by his public example (24) and testimony (25:8) it is clear Paul never did teach it was appropriate to neglect physical circumcision as an act of obedience to God.
If Paul didn’t discourage Jewish believers from circumcising their children, he wouldn’t have discouraged Gentiles from doing so either; circumcision was not a particularly Jewish thing (Jn 7:22); it existed in Abraham, the father of us all (Ro 4:16), long before the Jewish people.
So, Paul encourages circumcision in the context of obedience to God’s Law and forbids it when it is embedded in the larger context of ritual conversion to Judaism. Further, Paul teaches it is unnecessary for Jews to renounce their Jewishness by undergoing a formal act of becoming uncircumcised (1Co 8:18a), and encourages Gentiles to retain their ethnic and national identity rather than becoming Jewish. (18b) Effectively, he sees national identity as irrelevant in the context of defining a right relationship with God (19a); what’s important is staying true to the gospel while faithfully keeping God’s commands. (19b)