Rom 9:7

“Paul wants to say that it is not those who are of Abraham's flesh who are his children, but those who are of the promise, who are godly and just, whom God promised according to his foreknowledge would be children of Abraham, just as Isaac was made righteous by the promise.”

“"Neither, because they are the seed of Abraham, are they all children." Now when you come to know of what kind the seed of Abraham is, you will see that the promise is given to his seed, and know that the word hath not fallen to the ground. Of what kind, pray, is the seed then? It is no saying of mine, he means, but the Old Testament itself explains itself by saying as follows, "In Isaac shall thy seed be called." What is, "In Isaac?" Explain.”

“Not all Jews are children of Abraham, but some still are; and if not all Israelites are from Israel, then some … are from the Gentiles. Even so, the sons of Abraham were named in Isaac alone and not in Ishmael, although he too descended from Abraham's line.”

“Although it was beyond the capacity of nature, Abraham became a father by divine generosity. Paul says this, even though Ishmael was also Abraham's son and moreover, he was the firstborn. Therefore why do you boast, O Jew, that you are the only one to be descended from Abraham? For if you think that Ishmael does not count because he was the son of a slave, you are wrong. Holy Scripture reckons descent through the father and not through the mother. After all, the holy apostle could have mentioned the children born to Abraham through Keturah and shown that although they were born to a free woman they were not recognized as Abraham's seed. It would also have been easy for Paul to show that the twelve sons of Jacob had different mothers, and four of them were the children of slaves, yet all of them belonged to Israel, and none of them was hurt by his mother's slavery.… Here Paul wanted to insist that it was not the entire race of Abraham which received the blessing. Rather, only one of his sons was blessed, and the others were rejected.”

“God, he says, fulfilled what was promised. He said: "To you I will give it, and to your offspring" (Gen. 13:15). So then, let us see what this offspring is. Not all who descended from Abraham are simply his offspring, and not all who descended from Israel are Israelites, but those born after the example of Isaac and distinguished by the virtue of Israel, through which he saw God. For he did not say: who are from Jacob, but: "from Israel," mentioning the more honorable name. Therefore, if you understand who is born after the example of Isaac, you will find that the promise is not false: for to such were the promises given. Therefore God by no means deserves reproach: what He said, He also fulfilled, even though some do not understand this.”

“Then he clarifies his statement, at but in Isaac. First, in regard to Abraham; second, in regard to Jacob, at and not only she. In regard to the first he does three things: first, he cites a text from Scripture, saying, but in Isaac will your seed be called. This the Lord said to Abraham, as it says in Genesis 21, when describing the expulsion of Ishmael. As if to say: not all who were born from Abraham according to the flesh belong to that seed to whom the promises were made, as it says in Galatians: to Abraham were the promises made and to his seed (Gal 3:16), but those who are like Isaac.”

“What Paul wants us to understand is that not all are worthy because they are children of Abraham, but only those who are children of the promise, that is, whom God foreknew would receive his promise, whether they are Jews or Gentiles.… Abraham believed and received Isaac on account of his faith, because he believed in God. By this the mystery of the future faith was indicated, that they would be brothers of Isaac who had the same faith by which Isaac was born, because Isaac was born as a type of the Savior by the promise. Thus whoever believes that Christ Jesus was promised to Abraham is a child of Abraham and a brother of Isaac. Abraham was told that all the nations would be blessed in his offspring. This happened not in Isaac, but in him who was promised to Abraham in Isaac, that is, Christ, in whom all the nations are blessed when they believe. Therefore the other Jews are children of the flesh, because they are deprived of the promise and cannot claim Abraham's merit, because they do not follow the faith by which Abraham is counted worthy.”