Commentaries:
Robertson's Word Pictures (NT)
The children of the promise (ta tekna thv epaggeliav). Not through Ishmael, but through Isaac. Only the children of the promise are "children of God" (tekna tou qeou) 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.
Other Robertson's Word Pictures (NT) entries containing Romans 9:8:
Romans 9:6
Romans 9:17
Romans 10:6
Romans 11:5
1 Corinthians 10:18
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