Some in the first century believed that Christ was only for the Jew, those who proudly claimed to be descendants of Abraham, and who also claimed proudly to be circumcised. Yet, Paul said the blessedness of forgiveness comes upon both circumcised and uncircumcised,
“Does this blessedness then come upon the circumcised only, or upon the uncircumcised also? For we say that faith was accounted to Abraham for righteousness. How then was it accounted? While he was circumcised, or uncircumcised? Not while circumcised, but while uncircumcised” (Rom 4:9-10).
The Lord blessed Abraham before he was circumcised, but later the Lord did tell Abraham to be circumcised, so that he was in both states, and since he is the father of the faithful, that makes him the father of both Jew and Gentile.