Rom 9:8

“"That is, they which are the children of the flesh, these are not the children of God: but the children of the promise, these are counted for the seed." 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 thy 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.”

“Not I, he says, am explaining to you what the true seed of Abraham is, but the Old Testament, which says: "In Isaac shall thy seed be called" (Gen. 21:12). Therefore those who were born after the example of Isaac, that is, according to the promise, are truly children of Abraham, and especially of God. For everything was accomplished by the word of God. Therefore, the word of God was fulfilled, but God granted what was promised to the true seed, that is, to the believers from among the Gentiles, who became children of God just as Isaac, because they too were of the promise. But if the Jews say that the words "in Isaac shall thy seed be called" mean that those born of Isaac are reckoned as the seed of Abraham, then one must also honor the Idumeans and all who descended from him, because their forefather Esau was a son of Isaac. But the Idumeans are not only not called sons of Abraham, but are even quite foreign to the Israelites and are called aliens.”

“Then he explains the quoted text so far as it applies to his thesis, when he says that is to say, not they who are the sons. To understand this it should be noted that the Apostle says in Galatians: Abraham had two sons, one by a slave and one by a free woman. But the son of the slave, namely, Ishmael, was born according to the flesh (Gal 4:22), because he was born according to the law and custom of the flesh from a young woman: the son of the free woman, namely, Isaac, through promise and not according to the flesh, i.e., not according to the law and custom of the flesh, because he was born from a sterile, old woman (Gen 18:10); although he was born according to the flesh, i.e., according to the substance of the flesh he received from his parents. From this the Apostle decides that those adopted into the sonship of God are not the sons of the flesh, i.e., not because they are the bodily descendants of Abraham, but they are accounted for the seed, to whom was made the promise, who are the sons of the promise, i.e., those who are made sons of Abraham because they imitate his faith, as it says in Matthew: God is able from these stones to raise up children to Abraham (Matt 3:9). Thus, Ishmael, born according to the flesh, was not numbered among the seed, but Isaac, born by the promise, was.”