Thomas Schreiner on Romans 9: Individual Election unto Salvation
Those interpreters who assert that Paul is referring merely to the historical destiny of Israel and not to salvation do not account plausibly for the relationship of verses 1–5 of the rest of the chapter, for verses 1–5 make it eminently clear that the reason Paul brings up the question of the faithfulness of God in verse 6 is that a great portion of Israel is not saved.
The subsequent context of in Romans 9:6b-29 also demonstrates that salvation is in view. For example, Paul argues that mere ethnic descent from Abraham does not make anyone a child of God (9:6b-9). It is the children of the promise who are truly the children of God. The phrases children of God (tekna tou theou, 9:8) and children of the promise (tekna tes epangelias, 9:8) always refer in Paul’s writings to those who are the saved children of God (cf. 8:16, 21; Phil. 2:15; Gal. 4:28).
Thomas R. Schreiner, Still Sovereign (pg. 92, emphasis original)