Now let’s get Paul’s take.
Paul was sorrowful and wished that he could be accursed and cut off from God for the sake of his Jewish “kinsmen according to the flesh“. This was a group with tremendous advantages over Gentiles because they had been entrusted with the oracles of God. (Romans 3:1-2) Jesus was a Jew. (Romans 1:3, Romans 9:5) Salvation was from the Jews (John 4:22) and Paul preached the gospel to the Jews first. (Acts 13:46)
Of this advantaged subset of Israelite brothers, he said:
“They are Israelites, and to them belong the adoption, the glory, the covenants, the giving of the law, the worship, and the promises. To them belong the patriarchs, and from their race, according to the flesh, is the Christ, who is God over all, blessed forever. Amen.” (Romans 9:3-5)
The Jews (Judah, Benjamin and some Levites) had maintained their identity, language, God’s law and a lineage that could be traced back to Abraham. But they still only represented roughly two of the tribes. We know from the New Covenant that all TWELVE tribes would be reunited. This must be the lense through which we interpret Paul when he speaks of one body.
“For just as the body is one and has many members, and all the members of the body, though many, are one body, so it is with Christ. For in one Spirit we were all baptized into one body—Jews or Greeks, slaves or free—and all were made to drink of one Spirit.” (1 Corinthians 12:12-13)
“Therefore remember that at one time you Gentiles in the flesh, called “the uncircumcision” by what is called the circumcision, which is made in the flesh by hands— remember that you were at that time separated from Christ, alienated from the commonwealth of Israel and strangers to the covenants of promise, having no hope and without God in the world. But now in Christ Jesus you who once were far off have been brought near by the blood of Christ. For he himself is our peace, who has made us both one and has broken down in his flesh the dividing wall of hostility by abolishing the law of commandments expressed in ordinances, that he might create in himself one new man in place of the two, so making peace, and might reconcile us both to God in one body through the cross, thereby killing the hostility. And he came and preached peace to you who were far off and peace to those who were near. For through him we both have access in one Spirit to the Father. So then you are no longer strangers and aliens, but you are fellow citizens with the saints and members of the household of God, built on the foundation of the apostles and prophets, Christ Jesus himself being the cornerstone, in whom the whole structure, being joined together, grows into a holy temple in the Lord. In him you also are being built together into a dwelling place for God by the Spirit.” (Ephesians 2:11-32)
“When you read this, you can perceive my insight into the mystery of Christ, which was not made known to the sons of men in other generations as it has now been revealed to his holy apostles and prophets by the Spirit. This mystery is that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel.” (Ephesians 3:4-6)
“There is one body and one Spirit—just as you were called to the one hope that belongs to your call— one Lord, one faith, one baptism, one God and Father of all, who is over all and through all and in all.” (Ephesians 4:4-6)
“For the husband is the head of the wife even as Christ is the head of the church [ekklēsia], his body, and is himself its Savior.” (Ephesians 5:23)
“And he is the head of the body, the church [ekklēsia]. He is the beginning, the firstborn from the dead, that in everything he might be preeminent.” (Colossians 1:18)
“Now I rejoice in my sufferings for your sake, and in my flesh I am filling up what is lacking in Christ’s afflictions for the sake of his body, that is, the church, [ekklēsia]” (Colossians 1:24)
“Do not lie to one another, since you laid aside the old self with its evil practices, and have put on the new self who is being renewed to a true knowledge according to the image of the One who created him— a renewal in which there is no distinction between Greek and Jew, circumcised and uncircumcised, barbarian, Scythian, slave and freeman, but Christ is all, and in all.” (Clossians 3:9-11)
“Let the peace of Christ rule in your hearts, to which indeed you were called in one body; and be thankful.” (Colossians 3:15)
In case you haven’t seen it by now, God has ONE people and they are known by many names: The Redeemed, the ekklēsia [translated as assembly, congregation or Church], the Bride of Christ, the Body of Christ, those who are Christ’s, the elect, the offspring of Abraham, the children of promise, a treasured possession, a kingdom of priests, a holy nation, a peculiar people and the Israel of God. These are all descriptions of the same assembly.
Isaiah prophesied that God would pour out his Spirit in a way that would result in some naming themselves by the name of Israel.
“For I will pour water on the thirsty land, and streams on the dry ground; I will pour my Spirit upon your offspring, and my blessing on your descendants. They shall spring up among the grass like willows by flowing streams. This one will say, ‘I am the Lord’s,’ another will call on the name of Jacob, and another will write on his hand, ‘The Lord’s,’ and name himself by the name of Israel.” (Isaiah 44:3-5)