Thomas Aquinas, on Rom 9:8

“That is to say, not they that are the children of the flesh, are the children of God; buy they, that are the children of the promise, are accounted for the seed.”

“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.”

Imported from an open dataset — not yet checked against the printed edition.