1. The names "Jacob" and "Israel" are interchangeable. Jacob-- the son of Isaac, and grandson of Abraham -- was originally named Jacob (Supplanter) but God renamed him "Israel" (a prince with God and men) after Christ wrestled with him.
2. Later Jacob and/or Israel meant the descendants of Jacob -- the twelve tribes of Israel.
3. "All Israel is not Israel" because the majority of Jews rejected Christ, and there was only a small believing "remnant". That will also be true in the future, but "small" here is a relative term.
4. The modern nation of Israel has only a very small number of Sephardic Jews (descended from the 12 tribes). The majority of Israelis are Ashkenazim (European Jews or those from Central Asia (from the Khazars).
5. "Spiritual Israel" is simply believing Jews, but God sees them as the "good olive tree" into which the Gentiles have been "grafted" as branches from "the wild olive tree". This is in fact the Church (redeemed Jews and Gentiles in to Body of Christ). This Paul calls the Church "the Israel of God", and God has a plan and purpose for the Church in eternity.
6. At the same time, God also has a plan for redeemed and restored Israel (the 12 tribes) on earth after the Second Coming of Christ. See Romans 11:25-36 (called "Jacob" in this passage). So the Church does not replace Israel, and Israel on earth has been promised all the land between the Nile and the Euphrates through the Abrahamic Covenant. This will become a reality in the future,
Not all of the Israel nation have the promise of an eternal inheritance of heaven, part of them do, because God's redeemed children consist of people from every kindred, and tongue, and people, and nation, and they will be reigning on earth until Jesus returns, in the air, at his second coming to have his angles to gather them from everywhere on earth and take his kingdom, which is his bride, home to heaven. At that time the earth will burn with fire.
Spiritual Israel (Jacob) turned away from God causing God to blind their eyes, But God left in the midst of them an afflicted and poor people that shall trust in the Lord. The remnant of Israel shall not do iniquity, nor speak lies, neither shall a deceitful tongue be found in their mouth, for they shall feed and lie down, and none shall make them afraid (Zeph 3:11-13).
Blinded spiritual Israel, which is known as the lost sheep of the house of Jacob/Israel even though they are blinded, have not lost their promise of eternal heaven, and will have their eyes opened to see the truth before Christ returns by coming unto a knowledge of the truth, due to hearing and believing the teaching of the remnant.
A scripture, I believe, that harmonizes with this is Ezek 10, especially verse 10. The larger, outer wheel being blinded Jacob/Israel, and the smaller, inner wheel being the remnant.
I also believe this harmonizes with the two gates in Matthew 7. the wide gate being blinded Jacob/Israel, and the strait gate being the remnant of Jacob/Israel. Both gates hav the promise of eternal heaven.
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