Those being saved are always the chosen, whether from the Gentiles or from the Jews, and not every person from either race. "All Israel" may perhaps also mean, in the Apostle, all believers, whoever they may be, because they are truly Israel.
Blessed Theodoret understands it in this way: "By all Israel the Apostle means believers, whether they are Jews who have natural kinship with Israel, or Gentiles joined to Israel by kinship of faith."
"Since what the Apostle proclaims is important, he brings in as witness the Prophet who says the same. He does not offer testimony that blindness occurred, since this is evident. But that the Jews will believe and be saved is witnessed by Isaiah, who cries out and says: 'The Deliverer will come from Zion, and He will turn away ungodliness from Jacob'" (Chrysostom).
The Prophet Isaiah foretold the coming of Christ the Savior, calling Him the One Who delivers, or the Deliverer. He says that He will come from Zion, that is, from among the sons of Israel themselves, and will turn away ungodliness from Jacob, who was first called Israel.
To turn away ungodliness means to make one pious. It means to teach faith, to provide grace, to sanctify, and to present him to God as a sincere believer. In the apostolic times, ungodliness was turned away from many, many Israelites through faith in the Lord and the grace of the Holy Spirit.
It was turned away from many, but not from all. The mass of the people remained in ungodliness. Yet since the Prophet Isaiah decisively affirms, without limitation, that the Deliverer, when He comes, will turn away ungodliness from Jacob, and since this has not yet been fulfilled, it will certainly be fulfilled. The prophetic word cannot lie.
It will come to pass that ungodliness will be turned away from all Jacob. But since this cannot happen while he remains in blindness, there will be a time when this blindness falls away and Israel is saved, becoming truly pious.
This is what the Apostle wished to confirm by the prophecy he cited. But lest anyone interpret the Prophet's words crookedly and say that this turning away of ungodliness means that the Israelites will become completely faithful to the law given to them, rather than being sanctified by faith and grace, the Apostle cites other words of the same Prophet. These show wherein this turning away of ungodliness will consist.