I hear many Christian's even nearly cross the line of boasting, some even go over that line in my opinion. The argument these believers have in their opinion has Biblical basis but is imcomplete Biblical underatanding. Here is what many say, "The real remnant and true children of God are we who have faith, and Israel is not God's chosen." First part of such as statement is true but the second is error
To state the children of Israel are no longer chosen by God is false- but the part that is true is that those who are believing in the Messiah by faith are Abrahams real descendents. Yet the danger is the arrogance that the Apostle Paul warns in Romans 11
(Romans 11:17-21) But if some of the branches were broken off, and you, although a wild olive shoot, were grafted in among the others and now share in the nourishing root of the olive tree, do not be arrogant toward the branches. If you are, remember it is not you who support the root, but the root that supports you. Then you will say, “Branches were broken off so that I might be grafted in.” That is true. They were broken off because of their unbelief, but you stand fast through faith. So do not become proud, but fear. For if God did not spare the natural branches, neither will he spare you.
When Paul refers to the "natural branches" he is refering to Israel, God's original "chosen people" under the Old Mosaic Covenant called the Law.
The problem with the Law was not that it was bad but had no way to change mankinds internal sin problem. So the Christian who lives by faith do to the power of God's indwelling Holy Spirit have no basis for boasting, after all:
(Ephesians 2:8-9) For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast.
So a Christ follower under the New Covenant of Grace and The Spirit, through faith in the Lord Jesus Christ. The fact is we have no boasting rights in regards to salvation.
The truth is that originally Israel was God's means to reveal Himself as God to the other nations who were by technical definition "Gentile" yet although for a time Israel has not been in this position due as Paul would say, "unbelief" in their appointed Messiah, The Christ, yet this has not disqualified them entirely because many are prophesied to eventually come to the faith in their own Messiah, a descendent of Israel itself, (Jacob wh was later renamed Israel was the Son of Isaac, son of Abraham) who it was promised to in scripture "Through your seed, all the nations of the earth will be blessed."
We see the fulfillment of this through Jesus Christ who was a descendent of King David, who was a descendent of Abraham, Isaac and Jacob/Israel.
Yet in throughout the OT God had appointed Israel as his chosen, but when they failed to represent God and fell into idolatry, sin and became like the other nations God did send them into exile in Assyria/Babylonia but eventually restored them to the promised land he gave them. This appears symbolic, although it was a literal captivity and a literal later restoration to the land given them, the symbology is that these who are of Israel in faith receive their appointed Messiah by faith, they will also once again be the people of God. Not merely by natural descent but spiritual reinstatment. The returned once was to a earthly promised land yet when they who turn in faith to the Great King and Savior, they will again return to a Promised Land of Heavenly origins, not amongst the Caanenites of this world and the Philistines, Ammorites, Amelekites of sin, but a new heaven and new earth removed of evil.
(Revelation 21:9-27) Then came one of the seven angels who had the seven bowls full of the seven last plagues and spoke to me, saying, “Come, I will show you the Bride, the wife of the Lamb.” And he carried me away in the Spirit to a great, high mountain, and showed me the holy city Jerusalem coming down out of heaven from God, having the glory of God, its radiance like a most rare jewel, like a jasper, clear as crystal. It had a great, high wall, with twelve gates, and at the gates twelve angels, and on the gates the names of the twelve tribes of the sons of Israel were inscribed— on the east three gates, on the north three gates, on the south three gates, and on the west three gates. And the wall of the city had twelve foundations, and on them were the twelve names of the twelve apostles of the Lamb. And the one who spoke with me had a measuring rod of gold to measure the city and its gates and walls. The city lies foursquare, its length the same as its width. And he measured the city with his rod, 12,000 stadia. Its length and width and height are equal. He also measured its wall, 144 cubits by human measurement, which is also an angel's measurement. The wall was built of jasper, while the city was pure gold, like clear glass. The foundations of the wall of the city were adorned with every kind of jewel. The first was jasper, the second sapphire, the third agate, the fourth emerald, the fifth onyx, the sixth carnelian, the seventh chrysolite, the eighth beryl, the ninth topaz, the tenth chrysoprase, the eleventh jacinth, the twelfth amethyst. And the twelve gates were twelve pearls, each of the gates made of a single pearl, and the street of the city was pure gold, like transparent glass. And I saw no temple in the city, for its temple is the Lord God the Almighty and the Lamb. And the city has no need of sun or moon to shine on it, for the glory of God gives it light, and its lamp is the Lamb. By its light will the nations walk, and the kings of the earth will bring their glory into it, and its gates will never be shut by day—and there will be no night there. They will bring into it the glory and the honor of the nations. But nothing unclean will ever enter it, nor anyone who does what is detestable or false, but only those who are written in the Lamb's book of life.
The Lord Jesus Christ will take his throne permanently in Jerusalem in time and be once again the King, unlike in former years in Israel's history, these will accept God as their king and not say, "We want a king like all the other nations" no but as it is written:
"And the Lord said to Samuel, “Obey the voice of the people in all that they say to you, for they have not rejected you, but they have rejected me from being king over them. According to all the deeds that they have done, from the day I brought them up out of Egypt even to this day, forsaking me and serving other gods, so they are also doing to you. Now then, obey their voice; only you shall solemnly warn them and show them the ways of the king who shall reign over them.” (1 Samuel 8:7-9)
Yet one day in keeping with the prophesies in the Law, God will "circumcise the hearts" of the descendents of Israel, as it says:
"And the Lord your God will circumcise your heart and the heart of your offspring, so that you will love the Lord your God with all your heart and with all your soul, that you may live. And the Lord your God will put all these curses on your foes and enemies who persecuted you." (Deuteronomy 30:6-7)
These curses? The curses sin brings and the unrepentent in the end will be driven far from the presence of God forever, the fallen sons of Cannan, as well as the first Adam, the sinful nature and sin will be banished forever.
(Revelation 21:1-8) Then I saw a new heaven and a new earth, for the first heaven and the first earth had passed away, and the sea was no more. And I saw the holy city, new Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband. And I heard a loud voice from the throne saying, “Behold, the dwelling place of God is with man. He will dwell with them, and they will be his people, and God himself will be with them as their God. He will wipe away every tear from their eyes, and death shall be no more, neither shall there be mourning, nor crying, nor pain anymore, for the former things have passed away.” And he who was seated on the throne said, “Behold, I am making all things new.” Also he said, “Write this down, for these words are trustworthy and true.” And he said to me, “It is done! I am the Alpha and the Omega, the beginning and the end. To the thirsty I will give from the spring of the water of life without payment. The one who conquers will have this heritage, and I will be his God and he will be my son. But as for the cowardly, the faithless, the detestable, as for murderers, the sexually immoral, sorcerers, idolaters, and all liars, their portion will be in the lake that burns with fire and sulfur, which is the second death.”