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 you 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 your sake, since now it is not even this: for thou then were in need of bodily signs, but now there is no need of them. "And was it not possible," one might say, "from his faith to learn the goodness of his soul?" Yes, it was possible but you stood in need of this addition also. For since thou did not imitate the goodness of his soul, and were not able to see it, a sensible circumcision was given you, that, after having become accustomed to this of the body, you might little by little be led on to the true love of wisdom in the soul also, and that having with much seriousness received it as a very great privilege, you might be instructed to imitate and revere your ancestor. This object then had God not only in the circumcision, but in all the other rites, the sacrifices, I mean, and the sabbath, and feasts. Now that it was for your sake that he received the circumcision, learn from the sequel. For after saying that he received a sign and a seal, he gives the reason also as follows. That he might be the father of the circumcision— to those who received the spiritual circumcision also, since if you have only this (i.e. the carnal), no farther good will come to you. For this is then a sign, when the reality of which it is the sign is found with you, that is, faith; since if you have not this, the sign to you has no longer the power of a sign, for what is it to be the sign of? Or what the seal of, when there is nothing to be sealed? Much as if you were to show one a purse with a seal to it, when there was nothing laid up within. And so the circumcision is ridiculous if there be no faith within. For if it be a sign of righteousness, but you have not righteousness, then you have no sign either. For the reason of your receiving a sign was that you might seek diligently for that reality whereof you have the sign: so that if you had been sure of diligently seeking thereafter without it, then you had not needed it. But this is not the only thing that circumcision proclaims, namely righteousness, but righteousness in even an uncircumcised man. Circumcision then does but proclaim, that there is no need of circumcision.
Circumcision is meaningless if there is no faith within. It is a sign of righteousness, but if there is no righteousness, then there is no sign either. The reason the Jews received a sign was that they might seek diligently for the reality of which the sign spoke. If they had done so, they would not have needed the sign in the first place. Righteousness is not the only thing that circumcision proclaims; it also proclaims that righteousness can be found in an uncircumcised man (e.g., Abraham). In effect, therefore, circumcision proclaims that there is no need of circumcision.
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