Any who has faith in Christ is a child of Abraham, just as if he had been natural born, but more real.
Galatians 3:7 - Know ye therefore that they which are of faith, the same are the children of Abraham.
Galatians 3:29 - And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.
Galatians 4:28 - Now we, brethren, as Isaac was, are the children of promise.
Romans 2:28 KJV - For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh:
Romans 2:29 KJV - But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God.