All Israel shall be saved and He shall take away their sins. That’s what it says. The specifics as to how are not spelled out for us, that’s between Israel and God.
Obviously Paul understood that lots of Jews were already in hell at that point (for example, the rich man in Luke 16), and many more would come if they did not believe. Israel is differentiated from physical Israel in Romans 9, and other places, such as when Paul says that a Jew is one who is a Jew inwardly, circumcised in the heart, and not one who is only a Jew outwardly. Those Jews and Gentiles who are saved are the Elect, and those who are not saved are the reprobate.
Anyone who denies Jesus is the Christ has no part of heaven, and, therefore, the "all" can only mean all true Israel.