Romans 9:6

But it is not as though the word of God hath come to naught. For they are not all Israel that are of Israel.

Paul had not yet spoken plainly that Israel, through their rejection of Christ, was at that time itself rejected by God, although that thought dominated his mind. Before saying that unsayable thing, he would move to soften it by showing that what he was about to say did not apply to every Israelite. Paul stressed the fact that not all of Abraham's children were Jews, that some were associated with Israel who were not really Israelites in the true sense, and that such a condition had extended back all the way to Abraham, Ishmael not being counted as Abraham's seed at all, a fact which he would immediately stress.