Romans 4:12 : And the father of circumcision to them who not only are of the circumcision, but who also walk in the steps of that faith of our father Abraham which he had in uncircumcision. Paul does not use the term "father Abraham" as the Jews would use it, but he uses it in its Christian sense. He does not say that Abraham is the father of the circumcision. With Paul he is not the father of Jews as such, but only of those Jews who "walk in the steps of that faith of our father Abraham which he had in uncircumcision." He is the father of the believers, whether they be Gentiles or Jews. There is no difference; "for in Christ Jesus neither circumcision availeth anything, nor uncircumcision; but faith working through love" (Galatians 5:6). This accords with what Peter said in reply to the Judaizing teachers in Jerusalem: "And he made no distinction between us and them, cleansing their hearts by faith" (Acts 15:9).
In a national and fleshly sense Abraham was the father of the whole Jewish nation, but that is not the sense in which Paul here uses the term "father Abraham." God promised Abraham: "In thy seed shall all the nations of the earth be blessed" (Genesis 22:18). We do not have to guess to whom this promise refers, for Paul says: "Now to Abraham were the promises spoken, and to his seed. He saith not, and to seeds, as of many; but as of one, And to thy seed, which is Christ" (Galatians 3:16). Abraham's seed, through whom the world was to be blessed, was Jesus the Christ, and none other. But there is a sense in which all Christians are Abraham's seed. "And if ye are Christ's, then are ye Abraham's seed, heirs according to promise"(Galatians 3:29). In the high gospel sense contemplated in the promise made to Abraham, he is the father of only those who believe in Christ.
A Jew, as such, has no part in this promise. The spiritual fam-ily of Abraham has superseded the fleshly family. God's order is: the fleshly first, then the spiritual.