Really need to have studied the Old Testament Books also to understand what Paul said.
God has said that HE will have mercy, and forgive His chosen seed, and purify them, and gather them all, in final, even the lost ten tribes of Israel. The fact that God chose a specific 'seed' to represent His Salvation, and many of them rebelled through their history, does not mean He forgot them. This point Apostle Paul made when he said,
Rom 11:28-29
28 As concerning the gospel, they are enemies for your sakes: but as touching the election, they are beloved for the fathers' sakes.
29 For the gifts and calling of God are without repentance.
KJV
When Lord Jesus returns, everyone will understand why He has a 'chosen seed'.
Apostle Paul is one of the strongest examples of a 'chosen' one. Before Lord Jesus struck Paul (called Saul then) down on the road to Damascus, he was working for the Pharisees in hunting down Christians, and bringing them back to Jerusalem for trial. So Paul did not choose to become a Christian and believe The Gospel. Instead, Lord Jesus 'Divinely' influenced Apostle Paul's believe in Him, and forced Paul into His service. That is an example of a 'chosen vessel', which Lord Jesus called Paul. What all does that mean then, a chosen vessel?
In John 17, with Lord Jesus' prayer, He revealed more about His chosen Apostles. The Greek meaning for the word Apostle means 'to be sent'. They were thus 'chosen' and 'sent', and Jesus showed His Apostles were already owned by The Father when He gave them to Jesus. Like I said, all will better understand why this is when Lord Jesus returns in our near future.
We are not all 'chosen' sent ones like Christ's Apostles, nor even like the Old Testament prophets and patriarchs.
Many brethren desire to be a chosen Apostle, but that's not enough. And some, because they don't like the idea that 'some' are chosen 'sent ones', but they are not, bugs them like it's unfair, so they try to change what God Himself has ordained for this world.
Being 'called' only is not the same thing as being a 'sent one', an Apostle. Most of us are 'called' only, which is really what Ephesians 1 is about.
So all that above being understood, notice that Paul specifically showed that GOD HIMSELF put the
"spirit of slumber" upon those blinded Jews he was talking about...
Rom 11:7-11
7 What then? Israel hath not obtained that which he seeketh for; but the election hath obtained it, and the rest were blinded
8 (According as it is written, God hath given them the spirit of slumber, eyes that they should not see, and ears that they should not hear unto this day.
9 And David saith, Let their table be made a snare, and a trap, and a stumbling block, and a recompence unto them:
10 Let their eyes be darkened, that they may not see, and bow down their back alway.
11 I say then, Have they stumbled that they should fall? God forbid: but rather through their fall salvation is come unto the Gentiles, for to provoke them to jealousy.
KJV
How then, can those spiritually blinded Jews make their choice to believe on Jesus of Nazareth as Messiah with that "spirit of slumber" God put upon them? I mean, it's even ignorance on our part if we say they are already condemned for not believing of Jesus Christ today!
Likewise, how can one know that only 'some' of them will believe on Jesus, and not all of them, once that blindness is removed at Jesus' future coming???
So when Paul said all Israel will be saved, I'm not so sure that was just wishful thinking on his part. I believe Paul meant what he said based on what God promised the 'seed' of Israel in final.