25 Lest you be wise in your own sight, I do not want you to be unaware of this mystery, brothers: a partial hardening has come upon Israel, until the fullness of the Gentiles has come in. 26 And in this way all Israel will be saved, as it is written, “The Deliverer will come from Zion, He will banish ungodliness from Jacob”…28 As regards the gospel, they are enemies for your sake. But as regards election, they are beloved for the sake of their forefathers.
I have always wondered why the Jews do not believe in Jesus as the Messiah over the centuries. That is because they believe in only ONE advent of the Messiah to establish His glorious reign on earth which is prophesied in the OT. Even when our Lord appeared to the apostles speaking much about the Kingdom of God during the forty days after His Resurrection, the apostles still asked for the specific time that God will restore the kingdom of Israel. A clear evidence of the very consuming hope in the psyche of the Jews that one day, the Messiah will restore the Kingdom of Israel and reign in glory as prophesied in the Scriptures. Acts 1: 3-8.
(But Christians believe in 2 comings – the First Advent and the Second Coming – so that all prophecies of the initial suffering and the eventual glorious reign of the Messiah on earth are not contradicted.)
They can only believe in ONE advent of the Messiah in glorious reign. And not a crucified Messiah which is a stumbling block to them. 1 Cor 1:23.
So when the fullness of the Gentiles has come and Christ Jesus finally returns to reign on earth for 1000 years, the nation will see that the Messiah has finally come to reign and BELIEVE in Him. To them, it is the PROPHESIED COMING finally and not the first advent of a crucified Messiah.
I believe ALL ISRAEL does not mean every single Jew. It means the majority of the nation. They are still saved by faith alone in Christ Jesus.