In Paul’s theology, those who are united in Christ, whether circumcised Jews or uncircumcised Gentiles, are the true Israel in the eyes of God.
In Galatians 6: 12-16 Paul says the following:
As many as desire to make a fair shew in the flesh, they constrain you to be circumcised; only lest they should suffer persecution for the cross of Christ. For neither they themselves who are circumcised keep the law; but desire to have you circumcised, that they may glory in your flesh. But God forbid that I should glory, save in the cross of our Lord Jesus Christ, by whom the world is crucified unto me, and I unto the world. For in Christ Jesus neither circumcision availeth any thing, nor uncircumcision, but a new creature. And as many as walk according to this rule, peace be on them, and mercy, and upon the Israel of God.
Paul applies the title “the Israel of God” to those who have been reborn in Christ and not as a reference to ethnic Jews.
If the Israel of God mentioned in Galatians 6 were a reference to the ethnic Jews, then the cross has been made of no effect, for there would be two covenant families of God and not one. Furthermore, if the Israel of God is a reference to born again ethnic Jews, then there is still a dividing wall between Jews and Gentiles, and again, the cross is made of no effect.
In view of all Paul has written in Galatians, the Israel of God can only refer to believers in the Messiah, who, in the fullest sense of the word are Abraham’s seed and heirs according to the promise. Paul has made it abundantly clear that, in Christ Jesus, neither circumcision (which was the mark of Jewish national identity under the law) nor uncircumcision means anything. What matters is being a new creature, and those who are such are the true Israel in God’s sight.