Paul had Timothy circumcised:
Acts 16:1-3 (NASB)
1 Paul came also to Derbe and to Lystra. And a disciple was there, named Timothy, the son of a Jewish woman who was a believer, but his father was a Greek, 2 and he was well spoken of by the brethren who were in Lystra and Iconium. 3 Paul wanted this man to go with him; and he took him and circumcised him because of the Jews who were in those parts, for they all knew that his father was a Greek.
Numbers 15:15-16, "One ordinance shall be for you of the congregation, and for the stranger who sojourns with you, as an ordinance forever throughout your generations. As you are, so shall the stranger be in front of Yahweh. One Law and one manner shall be for you and for the stranger who sojourns with you."
Romans 9:6-8, "However, it is not as though Yahweh's plan had failed. For it is not everyone who is a descendant of Israyl who belongs to Israyl. Nor, just because they are his descendants, are they all Abraham's children; but: In Isaac will your seed be called. That is, it is not those who are the children of the flesh who are Yahweh's children; but it is the children of the promise who are regarded as Abraham's seed."