What does Romans 9:8 mean?
No Israelite would have disagreed with Paul's example in the previous verse. He pointed out that not all of Abraham's children were included in the line of God's promises to Israel. Only those who came through Isaac were included. So, while the promise was certainly given to Abraham's offspring, it was not promised to absolutely all his descendants.Now, though, Paul gets to the point of his illustration: It's not enough to simply be born "of the flesh" into Israel. It is the children of the promise who count as offspring. They are the ones who are truly Israel and will inherit God's promises to Abraham, Isaac, and Jacob. Paul will build from this argument to describe the difference between these two groups.
This parallels similar statements made by Jesus, who pointed out that being a true "child," spiritually, is not a matter of biology. It's a question of one's spiritual state, and their relationship with God (John 8:36–39).