Charles Ellicott Commentary Romans 9:8

"That is, it is not the children of the flesh that are children of God; but the children of the promise are reckoned for a seed." — Romans 9:8 (ASV)

They which are the children.—The Apostle explains this restriction in a spiritual sense. Mere natural descent gives no claim to membership in the theocracy.

Of the promise—i.e., not merely “promised children,” but “children born through the miraculous agency of the promise;” the promise is regarded as being possessed of creative power. (Compare to Romans 4:18-20.)