Albert Barnes on Romans 4:12

And the father of circumcision, the father, i.e., the ancestor, exemplar, or model of those who are circumcised, and who possess the same faith that he did. Not only the father of all believers, (Rom 4:11) but, in a special sense, the father of the Jewish people. In this, the apostle intimates that though all who believed would be saved as he was, yet the Jews had a special proprietorship in Abraham; they had special favours and privileges from the fact that he was their ancestor.

Not of the circumcision only. Who are not merely circumcised, but who possess his spirit and his faith. Mere circumcision would not avail; but circumcision, connected with faith like his, showed that they were peculiarly his descendants. Rom 2:25.

Who walk in the steps, etc. Who imbibe his example; who imbibe his spirit; who have his faith.

Being yet uncircumcised. Before he was circumcised. Comp. Gen 15:6, with Gen 17.