Circumcision was and remains one of the most important markers of Jewish identity. It testifies both to Israel’s trust in God regarding its future and to God’s promise justifying that trust.
The question, therefore, is indeed a significant one: Why did the Apostle Paul oppose circumcision for Gentiles? After all, he expressed this opposition while professing to be a true Pharisee! (Acts 23:6) Yet this occurred after his encounter with the risen Jewish Christ.
The answer is not as complicated as it may first appear.
Apostle Paul, as most Jews during his lifetime, used the word “circumcision” as a code word for Jewish identity (Col. 4:11). While he thought that being a Judean was an advantage in many ways (Rom. 3:1-2), he still adamantly opposed Gentile proselyte conversion to Judaism. His reasoning resulted from a belief that something significant had happened – Gentiles had also become recipients of the gift of the Holy Spirit. Those receiving this gift from God were not converts to Judaism, but rather Gentile God-fearers not attempting to become Jews (Eph. 3:6; Acts 15:7-8).
If all Gentile Christ-followers were to go through the proselyte conversion and become Jews in every way, it would sabotage God’s cosmic plan of revealing himself to the world. Paul’s Pharisaic idea was simple: Judeans should stay Judeans and nations should stay nations. Both must unite in worship of the one true God (1 Cor. 7:17-20).
Why was Paul against circumcision? Because this true Jew held a strong conviction that the glory of Israel’s God must become known to the entire world! His God could not be the God of the Jews only, for he is simply too big for that (Rom. 3:29).