Romans 4:12

and the father of circumcision to them who not only are of the circumcision, but who also walk in the steps of that faith of our father Abraham which he had in uncircumcision. [Now, circumcision was not given to Abraham to justify him, but as a seal, or token, that he had obtained righteousness by faith. Moreover, it was given to him that he might become the father of all believing Gentiles, God having agreed to make him the head or spiritual father of all those saved by Christ on condition of his being circumcised, and Abraham having been circumcised in order to obtain this exalted honor, and thirdly, that he might be the spiritual father of those who are not only circumcised like him, but walk in the steps of that faith of his of which circumcision was the seal. Thus circumcision was the seal that God had made Abraham the father of all who believe in God, and are justified by their belief, whether they belong to the Jews, who, in the earlier ages, had the better opportunity to believe, or to the Gentiles, who had that better opportunity in these latter ages.]