Looked at after the fact, his gospel is an astounding achievement:  transforming the most racially segrated religious doctrine into a universal and racially-inclusive faith.  And not by repudiating the Old Covenant ("the commandment is holy, righteous and good" Rom 7:12), but, as we have said, by reworking the sacred Old Testament tradition.  We see this in terms of two of the anchors of Jewish identity:

          a.  Abraham.  The religion of Judaism was coterminous with biological descent from Abraham.  (Actually, it was Jacob, not Abraham, who was the fount of Israel, because the offspring of both Ishmael and Esau were excluded.) Judaism was primarily a familial/racial religion.  The sons and daughters of Abraham were God's people, the inheritors of God's promises and His favor.  Jews believed they were a favored sub-species of humanity, set above the common herd. Jesus warned his listeners not to rely on the biological factor:

           And do not think you can say to yourselves, ‘We have Abraham as our father.’ I tell you that out of these stones God can raise up children for Abraham (Mat 3:9).

But Paul went further:

          it is not the natural children who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring (Rom 9:8).

          For if the inheritance depends on the law, then it no longer depends on a promise; but God in his grace gave it to Abraham through a promise (Gal 3:18).

          If you belong to Christ, then you are Abraham’s seed, and heirs according to the promise (Gal 3:29).

This is extremely important.  Paul did not repudiate Abraham, but neither did he subjugate Gentiles to the dictates of biological descent.   Instead, he changed the terms of inheritance.  Participation in the New Covenant was through faith, not through genes.  In computer terms, (eg. "virtual reality")  one could be a "virtual" son of Abraham, without actually having a drop of Jewish blood in his veins.  And in God's eyes, this "virtual" sonship was not only as authentic as biological sonship, it actually displaced biology!  The sons of the Old Covenant were disinherited in favor of the "virtual" sons who had faith in Jesus.   This was the scandal of Paul's Gospel to the law-abiding Jews.
 

We have to remember that a Jew looked out on the world through a racial filter.  It was "us" and "them."  "Them" was anyone who was not a properly credentialed and pedigreed Jew.    Paul was born and bred into this outlook.  And yet he could write,

            For through [Jesus] we both have access to the Father by one Spirit. Consequently, you are no longer foreigners and aliens, but fellow citizens with God’s people and members of God’s household (Eph 2:18-9).

We fail to grasp the radical nature of this insight in the history of human relations.  It is beyond anything in the Ancient World, and it surpasses the most egalitarian of modern political ideologies.  For he is not talking about ethnic realignments and self-determination, and each man/woman in their own homeland -- he is stating that being baptized into the Christian community destroys the very basis of political opposition and racial antagonism.  

Can you imagine a white Southern plantation-owner of the 1850's not only freeing his slaves, but inviting them into his mansion to live with him and eat at his table?   That is the distance Paul travelled in his own attitudes to Gentiles. The line of separation between races, which was a part of the foundation of social life, was abolished.  This metaphor of American slavery is not inaccurate, because Southern religion shared the blindness and offense of Old Covenant Jews.  Both groups vehemently loved their God, and in His Name erected an intransigent and impermeable racial barrier against their "inferiors."  And God help anyone who dared to puncture that barrier.  Paul was hated with the same fervor that later white segregationists hated civil rights advocates.