Romans 2:27-28

27In that way, even Gentiles will show that you who are Jews are wrong. God's Law was written to help you Jews. They circumcised you to show that you belong to God. But if you refuse to obey God's Law, then God will punish you. The Gentiles who do obey God's Law will show that God is right to punish you.

28Not everyone who seems to be a Jew is really a Jew who belongs to God. God does not accept a man just because someone has circumcised that man's body.