Romans 4:12

If a Gentile who has not been circumcised follows Abraham's faith, which he manifested before being circumcised, he will not be rejected. For the God of all, since he had foreseen as God that he would gather one people from both Jews and Gentiles and …

Here again he lays low the lofty spirit of the Jews by reminding them of the time of the justification. And he well says, "the steps," that you as well as Abraham may believe in the resurrection of bodies that are dead. For he also displayed his faith upon this point. And so if you reject the uncircumcision, be informed for certain that the circumcision is of no more use unto you. For if you follow not in the steps of his faith, though you were ten thousand times in a state of circumcision, you will not be Abraham's offspring. For even he received the circumcision for this end, that the man in a state of uncircumcision might not cast thee off. Do not then demand this of him too. For it was you whom the thing was to be an assistance to, not he. But he calls it a sign of the righteousness. And this also was for thy sake, since now it is not even this: for thou then wert in need of bodily signs, but now there is no need of them.

But as to the fact that after the faith which he had in uncircumcision he received circumcision, he consequently sets forth the reason for which circumcision was given him. It was, he says, that it might be the seal of that faith which he had while placed in uncircumcision, so that through this "he might be the father of all who believe through uncircumcision, that it might be reckoned to them unto justice, and that he might be the father of circumcision, not only to those who are of the circumcision, but also to those who follow the footsteps of the faith of our father Abraham, which he had in uncircumcision." But if Abraham, while he was still in uncircumcision, was justified by faith, he can become also the father of those who are born in circumcision, provided that they belong to that faith which justified him while placed in uncircumcision. And since it justified Abraham while placed in uncircumcision, he is therefore made the father of both kinds: of those who are in uncircumcision through faith, and of those who are in circumcision through the flesh; and after circumcision he begets first Isaac, and from him takes the beginning of the ogdoad, that is, in him the first mysteries of the eighth day are consecrated, and from that point the posterity of Abraham begins to run by a twofold sacrament. For just as faith is understood by its own nature to be one thing, and the justice which was reckoned to him through faith another; so also his posterity in uncircumcision is one thing, and his posterity in circumcision another. […]

Paul says this because Abraham by believing became the forefather of the circumcision, but of the heart, not only of those who descended from him but also of those who, from among the nations, believed in the way he did. He is the father of …

Then he shows the manner in which he is the father even of the uncircumcised, namely, by imitation. And this is what he says: that he might be the father . . . not to those only who are of the circumcision, but to those …