"And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob."
The context of this chapter makes it very plain that Paul was not talking about Israel after the flesh. Who is the Israel who will be saved? Paul had just finished spelling it out in verses 16-25. He described how the Gentiles would be grafted into the olive tree representing the Jewish people. As the Gentiles (represented by the wild olive tree) were grafted in, they began to partake of "the root and fatness" of the Israelites (verse 17). The "natural branches," or Jews (verse 21), were cut off because of unbelief, and believing Gentiles were accepted as spiritual Israel.
In Galatians 3:29, Paul said, "And if ye be Christ' s, then are ye Abraham's seed, and heirs according to the promise." The picture grows clearer still as we read Romans 9:6-8. "For they are not all Israel, which are of Israel: Neither, because they are the seed of Abraham, are they all children: .... That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed."
So it is faith that makes one a spiritual Israelite, not the accident of physical birth. It is the new birth that places Jew and Gentile into the spiritual family of God, all of whom will be saved.