Heb 8:7 For if that first covenant had been faultless, then no place would have been sought for a second.
Heb 8:8 Because finding fault with them, He says: "BEHOLD, THE DAYS ARE COMING, SAYS THE LORD, WHEN I WILL MAKE A NEW COVENANT WITH THE HOUSE OF ISRAEL AND WITH THE HOUSE OF JUDAH—
Heb 8:9 NOT ACCORDING TO THE COVENANT THAT I MADE WITH THEIR FATHERS IN THE DAY WHEN I TOOK THEM BY THE HAND TO LEAD THEM OUT OF THE LAND OF EGYPT; BECAUSE THEY DID NOT CONTINUE IN MY COVENANT, AND I DISREGARDED THEM, SAYS THE LORD.
Heb 8:10 FOR THIS IS THE COVENANT THAT I WILL MAKE WITH THE HOUSE OF ISRAEL AFTER THOSE DAYS, SAYS THE LORD: I WILL PUT MY LAWS IN THEIR MIND AND WRITE THEM ON THEIR HEARTS; AND I WILL BE THEIR GOD, AND THEY SHALL BE MY PEOPLE.
Those who claim that Christianity is and should be a gentile church are not, as you point out, recognizing that they have been grafted into the tree of Israel only through the grace of God and the shed blood of Christ.
When Christ died on the Cross, He was the New covenant with believers. Only believers. Those who do not believe in Christ ,as the Son of God , go straight to the grave and oblivion. The only way to eternal life is stated in John 3:16. The “chosen people” are believers in Christ as the Son of God.The old testament is completely dead.The Bible I read is New Testament only. Those that rejected Christ are to be buried and forgotten. They do not get an eternal existence. NOT AT ALL!!
Paul Johnson speculats that more Jews came into the Church than we has thought. Fact is that the Jews in the West owed less to the Jews of Mesopotamia than this win the Levant and Anatolia. In Alexandria, Rome, Gaul and Spain the doctrine of the Pharisees was less well know,and Hebrew less spoken, and so more open to the Gospel.
Fact is we have only a very limited idea of how Christianity spread during the first generation.