Paul begins exploring his major issue — God’s faithfulness and the place of Israel in God’s plan — by saying this: If you look at it, it has never really been just “flesh” that makes a person part of “Israel.” No, in fact, it is the Promise Line that is the true Israel.
His major statement is this: “Not all who are from Israel are in fact Israel” (635): Romans 9:6. Cleverly, Wright connects this to the two “I”s of Romans 7. Not all of Abraham’s children are his “seed” (or true descendants).
It was through Isaac; then Jacob. Not just descendants of Abraham.
So, the claim of Paul that there is a Messianic line and a non-Messianic line has precedent; it is at the heart of Israel’s story.
Israel’s story is a story of grace and tragic failure; Israel was regularly refined to a remnant. So, God’s story is has been like this all along. God’s promise, in other words, has been elective. (He says this is not directly about individuals but about Israel’s story.)