Gentiles who believe are not Jews either as being members of the remnant or by being in the body of Christ.
Maybe if we look at the passages that speaks about all becoming "one new man" would bring clarification? If we are "One new man," then would a True Jew be the same as a True Christian? And wouldn't a True Christian be a True Jew? Paul was Truly Jewish, but didn't he consider himself to be a Christian?
Ephesians 2:14-16 CSB - "For he is our peace, who made both groups one and tore down the dividing wall of hostility. In his flesh, 15 he made of no effect the law consisting of commands and expressed in regulations, so that he might create in himself one new man from the two, resulting in peace. 16 He did this so that he might reconcile both to God in one body through the cross by which he put the hostility to death."
Romans 2:27-29 CSB - "A man who is physically uncircumcised, but who keeps the law, will judge you who are a lawbreaker in spite of having the letter of the law and circumcision. 28 For a person is not a Jew who is one outwardly, and true circumcision is not something visible in the flesh. 29 On the contrary, a person is a Jew who is one inwardly, and circumcision is of the heart -- by the Spirit, not the letter. That person's praise is not from people but from God."
I may be wrong, but Paul seems to be teaching that being Jewish by blood means nothing. However, to be Truly "Jewish", simply means to have a Right Heart, produced by God's Spirit, and this has been happening from the beginning and into today.
Ezekiel 11:19-20 CSB - "I will give them integrity of heart and put a new spirit within them; I will remove their heart of stone from their bodies and give them a heart of flesh, 20 "so that they will follow my statutes, keep my ordinances, and practice them. They will be my people, and I will be their God."
What we see, here, in Ezekiel (and perfectly mirrored again in Eze 36) is what it means to be a True person in relationship with God, and it has nothing to do with being Jewish.
It is an amazing Bible with many Mysteries. I wish we all saw the same Plan of God.