12 And the father of circumcision to them who are not of the circumcision only, but who also walk in the steps of that faith of our father Abraham, which he had being yet uncircumcised.
After believing, Abraham received circumcision as a sign of the faith which he already had … which is how he can also be the father of those who are circumcised, provided that they share the same faith which he had while he was still uncircumcised.
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 the Jews according to the flesh, but according to faith he is the father of all believers.
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.
Therefore all the Gentiles who believe are children of Abraham when faith alone is credited to them as righteousness and they too receive the circumcision—of the heart. Or perhaps this verse should be understood like this: Abraham was righteous in uncircumcision so that he could become the father of uncircumcised believers, and he remained righteous once he was circumcised in order to become the father of the righteous who are circumcised.
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 prepare for them salvation by faith, had appointed Abraham as their father as well. Paul had demonstrated that Abraham had acquired righteousness by faith before being circumcised and that even after being circumcised he lived not according to the law of Moses but by clinging to his faith. Hence Paul called him the father of the Gentiles who believe.
[AD 202] Irenaeus on Romans 4:12