John Calvin on Romans 9:8

8. That is, They are not, etc. He now gathers from God’s answer a proposition, which includes the whole of what he had in view. For if Isaac, and not Ishmael, was the seed, though the one as well as the other was Abraham’s son, it must be that all natural sons are not to be regarded as the seed, but that the promise is specially fulfilled only in some, and that it does not belong commonly and equally to all. He calls those the children of the flesh, who have nothing superior to a natural descent; as they are the children of the promise, who are peculiarly selected by the Lord.