Notice in Romans 9:2-4, Paul talks about his sorrow over those of his own race:
9:2-4 “I have great sorrow and unceasing anguish in my heart. For I could wish that I myself were cursed and cut off from Christ for the sake of my brothers, those of my own race, the people of Israel. Theirs is the adoption as sons; theirs the divine glory, the covenants, the receiving of the law, the temple worship and the promises.”
Why do you suppose he was in such distress if “all Israel will be saved” (as dispensationalists interpret the phrase)?
Again, just two verses later:
Romans 9:6: “It is not as though God’s word had failed. For not all who are descended from Israel are Israel.”
Paul was distraught over those of his own race rejecting Christ and he made the point that their bloodline didn’t make them Israel.
I think Paul is saying that a Jew is a Jew from the heart, from the inside out, who values his heritage, knows where he came from, and followed in the ways of his people -- not just someone who had Jewish blood. Many back then and throughout history have been Jewish in name and blood only and cared little about Torah, and Moses, and Isaiah, their heritage, their covenants, and the promises to them, the things that made them uniquely Israel before God.