He received the sign of circumcision as a seal of the righteousness that he had by faith while he was still uncircumcised. The purpose was to make him the father of all who believe without being circumcised, so that righteousness would be counted to them as well,
11. And he received the sign, etc. In order to anticipate an objection, he shows that circumcision was not unprofitable and superfluous, though it could not justify; but it had another very remarkable use, it had the office of sealing, and as it were of ratifying the righteousness of faith.
Abraham received justification by faith fourteen or twenty-five years before circumcision, silencing the fears of the uncircumcised Gentiles and humbling the pride of the Jews who boasted in their circumcision as if it held a monopoly on blessing. Circumcision came only afterward as a seal and confirmation of righteousness already imputed.
AI summary
Commenting on Romans 4:9-16
And the father of circumcision,.... So the Jews call Abraham (l), "the head of those that are circumcised"; and (m), "the head to them that are circumcised"; but the apostle here says, he is a father to them who are not of the circumcision only; not to the Jews only, in a spiritual sense, and not to all of them, since some were "of Israel"...