There is only one people of God, going all the way back to Abraham, of both OT and NT saints, the church.
And all who believe become a part of the church, including all Jews.
There are not two peoples of God, on two tracks, with two destines.
That is contrary to everything the apostles teach.
IF they do not persist in unbelief, Israel will be grafted back into the one olive tree of God's people (Ro 11:23), the church, going all the way back to Abraham.
God has only one people, those saved by faith in Jesus Christ, of both OT and NT saints,
OT saints who believed in the promise (Ge 15:5, seed; Jesus Christ,Gal 3:16), and
NT saints who believe in Jesus Christ.
All this is true, we are ONE in Christ by Faith, but you are missing the thrust of the overall point. Maybe I can reduce it to a common number so to speak. We as [Gentile mostly] Christians are not all given the same calling, even though we all come unto Christ by faith alone. Israel's calling does not have to be the Gentile churches calling either, even though God clearly is not going to accept their repentance, except it come by faith alone. That is why, as you say, Paul says that if (when) Israel believes again they will be grafted back in. But, they only do this after the Pre Trib Rapture or time of the Gentiles comes full. Jesus rules in the Kingdom Age only with Jewish humans, and with Gentiles Martyrs who refuse to take the Mark of the Beast (Rev. 20:4 states this, he only needs so many of us with Glorious bodies on earth I guess). The best way for me to explain the separate the callings
(Notice, I don't say we come unto God in a different manner, God just sees fit to give us different callings) is to show it in each of the Seven Feasts of God. Who can question the potter ?
The 7 Feasts of God were really just God having Israel practice future events that the Messiah would fulfill. They were called Holy Convocations (Dress Rehearsals) and thus Israel, unknowingly were showing the future world what was soon to come (well, in the next few thousand years). Remember, Jesus has to fulfill each Feast.
The Spring Feasts
1.) Feast of Passover (Jesus' blood is our Passover, he died on the exact day and time needed, therefore Fulfilled)
2.) Feast of Unleavened Bread (Jesus was unleavened, or without Sin, so this was fulfilled by Jesus)
3.) Feast of First-fruits (Jesus was the First-fruits of the Grave so this was fulfilled by Jesus)
Summer Harvest/Church Age
4.) Feast of Weeks/Pentecost/Church Age/Summer Harvest ( Jesus is fulfilling this now as we speak, he is our High Priest in heaven and we are harvesting souls for God and the kingdom during the Church Age. Notice, this feast was all alone on the calendar, and just like the Gentile Church Age is the only feast not brought to pass under the "Jewish times" of service unto God. What ended the Summer harvest every year? The Feast of Trumps, the whole no man knows the day nor hour comes from this,
Israel were on Lunar time (God's time/Appointed times) so they never knew the exact day nor hour the New Moon would come in, so they sent two men u into the mountains to spy out the moon phases, when the new moon came in they sent word back, and the Jewish leaders commenced to blowing the Shofar or Trumps. They blew in 9 sets of 11 or 99 times, then on the
"LAST TRUMP" they blew it longer & louder than the other 99 and this officially ended the Sumer Harvest
(THINK Church Age ends via Jesus' Trump in Rev. 4:1).
Fall Feasts Jesus is yet to fulfill
5.) Feast of Trumps (Jesus fulfills this when he called his Bride the Church home, then Israel is back on the clock, it is God time, He alone appoints these things, way ahead of what we understand. Even the last two Feast are also very evident, they are the appointed times of God. Jesus fulfills this by calling us home in Rev. 4:1 via the Last Trump.)
6.) Feast of Atonement (Who needs to atone before Daniels 70th week can end? Israel, when do they repent? Well, Zech. 13:8-9 shows that 1/3 repent, and that the very next verse show the DOTL arrives as Zech. 14:1 shows us. So, the 7 Feasts showed us the whole history of the world, even the SEPERATED Gentile Church Age, which came in between the Three Spring Feasts Jesus fulfilled. Of course Jesus' blood is what atones their sins, thus he fulfills this.)
7.) Feast of Tabernacles (to TABERANACLE simply means to dwell with God, and since Jesus rules from Jerusalem for 1000 years in the Kingdom Age with his Jewish brothers, I would sat that is fulfilled by Jesus also.)
So, God of course gave the Jews a CALLING to birth the Messiah, they should have also BELIEVED and taken the Gospel of the Kingdom unto the whole world, but they FAILED, so God called another people He had not heretofore "known" [as His people]. Thus our calling as the Church was specifically to take the Gospel unto all the nations, soo to be fulfilled !! God's calling of a Kingdom Age is promised unto Jews only, not Gentiles, we will be in Heaven, with God (probably finishing off the New Jerusalem, after all as it descends it is called the Bride of Christ). Rev. 20:4 specifically says only those Martyrs who refuse the Mark of the Beast, thus no one not living on earth during the 70th week can qualify.
We are not that different, you just allow God's ability to call different people for different purposes to escape you when it comes to His ability to do that for nations. Pssstt, Israel married the Father long ago, they do not need to marry Jesus, they have the Fathers name on their forehead, the 144,000 is simply a code for ALL Israel who repents, just like the Woman is in Rev. 12. Jacob had two brides, the beloved in Rachel and the forced one in Leah. Individual Jews who come to Christ during the Church age will wed Jesus. Those Saints of old and those who repent during the 70th week, 3.5-5 million Jews, not 144,000 (THINK CODE as in 12 = Fulness 10 = Completeness and times it all out, it goes like this, 12 x 12 x 10 x 10 x 10 or ALL Israel who repents) will have the Fathers name on their foreheads.