1) In Romans 9 Paul makes the point that God’s word did not fail because all that are descended from Israel belong to Israel. He goes on to say it is NOT the children of the flesh that are the children of God, but it is the children of the promise. Paul is clearly separating the ethnic nation of Israel into two groups. The larger group is Israel by birth but they are not saved and do not inherit the spiritual promises made to Abraham. This group does not belong to Israel according to Paul and are not the children of promise. Paul even uses the examples of the twin brothers of Jacob and Esau, Abraham’s grandchildren, to make this point of distinction. The children of the promise that inherit spiritual salvation are spiritual Israel. Therefore, according to Paul God’s spiritual promises were for them only and his word did not fail. The distinction between spiritually saved Israel and unsaved Israel is clearly established in this passage.
2) It is also clearly established in Rom 2:28-29 where a Jew is described as being regenerated by circumcision of the heart and not just by being physically born a Jew. Here again Paul makes a distinction between spiritually saved Jews and unsaved Jews.
3) Does this category of spiritual Israel include Gentiles? Does it include the church aka the body of Christ made up of Gentile believers? Paul clearly says Gentiles are included in this category in Ephesians 3:6 where he says Gentiles are members of the same body: 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.
4) Galatians 3:28-29 confirms that Gentiles are included in spiritual Israel when Paul says: There is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for you are all one in Christ Jesus. And if you are Christ's, then you are Abraham's offspring, heirs according to promise.
5) In Galatians 6:16 Paul calls this body of believers the Israel of God….. It has to be a spiritual Israel he is referring to because it cannot possibly be unsaved ethnic Jews.
When we let the divinely inspired writers of the New Testament interpret the Old Testament and it's fulfillment in Jesus Christ, it is clear that there is only one spiritual people of God. They are in Christ through faith and is doesn't matter if they are Jew or Gentile. They all have the same destiny and that is a new heaven and a new earth..... quite a magnificent land promise!