And observe the judgment and depth of Paul's mind. For in interpreting, he does not say, "they which are the children of the flesh, these are not" the children of Abraham, but, "the children of God:" so blending the former things with the present, and showing that even Isaac was not merely Abraham's son. And what he means is something of this sort: as many as have been born as Isaac was, they are sons of God, and of the seed of Abraham. And this is why he said, "in Isaac shall your seed be called." That one may learn that they who are born after the fashion of Isaac, these are in the truest sense Abraham's children. In what way was Isaac born then? Not according to the law of nature, not according to the power of the flesh, but according to the power of the promise. What is meant then by the power of "the promise?"
Paul does not call them the children of Abraham, but rather “the children of God,” thus combining the past with the present and showing that even Isaac was not merely Abraham’s son. What Paul means is something like this: Whoever has been born in the way that Isaac was born is a son of God and of the seed of Abraham.… For Isaac was born not according to the law of nature nor according to the power of the flesh but according to the power of the promise.
Ishmael was born of a maidservant by sexual intercourse, but Isaac was begotten by supernatural means from old people, by God’s promise. So the promise, which Abraham’s faith merited, now makes Christians sons of Abraham, so that Abraham is indeed the father of many nations.
[AD 407] John Chrysostom on Romans 9:8