When Paul says that "all Israel shall be saved" (Romans 11:26) that needs to be understood in the light of Bible prophecy pertaining to the Abrahamic Covenant and the redemption and restoration of the kingdom of Israel under Christ and AFTER His Second Coming.
Since we know that only believing Israel is saved, and is called "the good olive tree" then "all Israel" means "all believing Israel". But it goes beyond that. It also means all the twelve tribes of Israel in the future.
After the Second Coming of Christ, only one-third of that vast number of Jews brought by God to the land of Israel will actually believe on the Lord Jesus Christ and be saved: And it shall come to pass, that in all the land, saith the LORD, two parts therein shall be cut off and die; but the third shall be left therein. And I will bring the third part through the fire, and will refine them as silver is refined, and will try them as gold is tried: they shall call on my name, and I will hear them: I will say, It is my people: and they shall say, The LORD is my God. (Zech 13:8,9)
Now this group of believing Jews will also belong to ALL TWELVE TRIBES and will be settled in the land of greater Israel (from the Nile to the Euphrates). And that is what "all Israel" means. The book of Ezekiel shows us that God will gather all the Jews from around the world and bring them to the land of Israel after the Second Coming of Christ.
EZEKIEL 48: THE TWELVE TRIBES IN THE FUTURE LAND OF ISRAEL
1 Now these are the names of the tribes. From the north end to the coast of the way of Hethlon, as one goeth to Hamath, Hazarenan, the border of Damascus northward, to the coast of Hamath; for these are his sides east and west; a portion for Dan.
2 And by the border of Dan, from the east side unto the west side, a portion for Asher.
3 And by the border of Asher, from the east side even unto the west side, a portion for Naphtali.
4 And by the border of Naphtali, from the east side unto the west side, a portion for Manasseh.
5 And by the border of Manasseh, from the east side unto the west side, a portion for Ephraim.
6 And by the border of Ephraim, from the east side even unto the west side, a portion for Reuben.
7And by the border of Reuben, from the east side unto the west side, a portion for Judah...
23 As for the rest of the tribes, from the east side unto the west side, Benjamin shall have a portion.
24 And by the border of Benjamin, from the east side unto the west side, Simeon shall have a portion.
25 And by the border of Simeon, from the east side unto the west side, Issachar a portion.
26 And by the border of Issachar, from the east side unto the west side, Zebulun a portion.
27 And by the border of Zebulun, from the east side unto the west side, Gad a portion...
Since we know that only believing Israel is saved, and is called "the good olive tree" then "all Israel" means "all believing Israel". But it goes beyond that. It also means all the twelve tribes of Israel in the future.
After the Second Coming of Christ, only one-third of that vast number of Jews brought by God to the land of Israel will actually believe on the Lord Jesus Christ and be saved: And it shall come to pass, that in all the land, saith the LORD, two parts therein shall be cut off and die; but the third shall be left therein. And I will bring the third part through the fire, and will refine them as silver is refined, and will try them as gold is tried: they shall call on my name, and I will hear them: I will say, It is my people: and they shall say, The LORD is my God. (Zech 13:8,9)
Now this group of believing Jews will also belong to ALL TWELVE TRIBES and will be settled in the land of greater Israel (from the Nile to the Euphrates). And that is what "all Israel" means. The book of Ezekiel shows us that God will gather all the Jews from around the world and bring them to the land of Israel after the Second Coming of Christ.
EZEKIEL 48: THE TWELVE TRIBES IN THE FUTURE LAND OF ISRAEL
1 Now these are the names of the tribes. From the north end to the coast of the way of Hethlon, as one goeth to Hamath, Hazarenan, the border of Damascus northward, to the coast of Hamath; for these are his sides east and west; a portion for Dan.
2 And by the border of Dan, from the east side unto the west side, a portion for Asher.
3 And by the border of Asher, from the east side even unto the west side, a portion for Naphtali.
4 And by the border of Naphtali, from the east side unto the west side, a portion for Manasseh.
5 And by the border of Manasseh, from the east side unto the west side, a portion for Ephraim.
6 And by the border of Ephraim, from the east side even unto the west side, a portion for Reuben.
7And by the border of Reuben, from the east side unto the west side, a portion for Judah...
23 As for the rest of the tribes, from the east side unto the west side, Benjamin shall have a portion.
24 And by the border of Benjamin, from the east side unto the west side, Simeon shall have a portion.
25 And by the border of Simeon, from the east side unto the west side, Issachar a portion.
26 And by the border of Issachar, from the east side unto the west side, Zebulun a portion.
27 And by the border of Zebulun, from the east side unto the west side, Gad a portion...
James clearly shows who the twelve tribes are below, "The Church" the Israel Of God.
James 1:1KJV
1 James, a servant of God and of the Lord Jesus Christ, to the twelve tribes which are scattered abroad, greeting.