Galatians 4.26 says “But the Jerusalem above is free, and she is our mother.” The “Jerusalem above” is Paul’s reference to Heaven, where Christ is at the right hand of the Father, and where believers who have died dwell with Christ right now. The striking thing about this verse is that the “Jerusalem above” is being contrasted with the present, earthly Jerusalem, which is a reference to ethnic Jews who are still in bondage to the Law.
If “earthly Jerusalem” is made up of ethnic Jews, it stands to reason that the “Jerusalem above”– the heavenly Jerusalem– is made up of spiritual Jews. And when Paul says “she is our mother” he is referring to all Christians– Jew and Gentile– being the children of that Jerusalem above. Therefore, this is one of the places the Bible teaches us that it is right to think of Christians, the Church, the Body of Christ, as spiritual Jews.
It is very biblical to conceive of the Church, because of our union with Christ, as the True Israel.