A.T. Robertson 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)

The children of the promise (τα τεκνα της επαγγελιας). Not through Ishmael, but through Isaac. Only the children of the promise are "children of God" (τεκνα του θεου) in the full sense. He is not speaking of Christians here, but simply showing that the privileges of the Jews were not due to their physical descent from Abraham. Cf. Luke 3:8.