Romans 9:7 - Matthew Poole's English Annotations on the Holy Bible

He had before made a difference of Israelites, and now he makes a difference of the seed of Abraham. This was ever and anon in the mouths of the Jews: We are Abraham's seed, 1 Thessalonians 8:33. But here he tells them, that all Abraham's seed were not the children of the promise; for it was said to Sarah, Genesis 21:12, that the promised seed should be confined to Isaac's line, of his issue should the Messiah come, and all the true seed of Abraham, who are born after the manner of Isaac, by the word and promise of God. And as Ishmael, though Abraham's natural seed, was cast out, and therein was a type of those who are born only according to the flesh; so Isaac is a type of Abraham's spiritual seed, who are born not of the power of nature, but by virtue of the promise of God.