"Now we, brethren, as Isaac was, are the children of promise. But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now. Nevertheless what saith the scripture? Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the freewoman. So then, brethren, we are not children of the bondwoman, but of the free." (Galatians 4:29-31)
This epistle was written to gentile saints in Galatia.
So also we, while we were children, were held in bondage under the elemental things of the world. 4 But when the fullness of the time came, God sent forth His Son, born of a woman, born under the Law, 5 so that He might redeem those who were under the Law, that we might receive the adoption as sons. (Galatians 4:3-5)
So we see that, again, Gentile believers are adopted into the New Covenant, which God made with the Jews, not Gentiles, and we are participants, not takers-over. God specifically singles out the "children of promise", who are descendants of the son of Sarah and who come to faith in Jesus Christ, as the true Israel.
Where do you find Gentile believers in this passage?
THROUGH ISAAC YOUR DESCENDANTS WILL BE NAMED. 8That is, it is not the children of the flesh who are children of God, but the children of the promise are regarded as descendants. 9For this is the word of promise: AT THIS TIME I WILL COME, AND SARAH SHALL HAVE A SON.
Was Sarah, who gave birth to the child of promise, a Jew or Gentile? Was her son, Isaac, a Jew or a Gentile? Who did God use to create those who He calls the "children of promise" Was it a Jew or a Gentile?