Gentiles could only join with Israel by conversion.
The mystery, the secret that was hidden, is that Gentiles can join in on the promises made to the children of Israel without conversion.
The mystery of Christ is the body of Christ which does NOT get "all of the promises made to the children of Israel".
However, the body of Christ and the children of Israel are both fellow heirs of the exact same kingdom that was promised to the children of Israel.
Nope.
When the kingdom comes, Israel will take its place as the head of the nations.
Rev 21:9-14 KJV And there came unto me one of the seven angels which had the seven vials full of the seven last plagues, and talked with me, saying, Come hither, I will shew thee the bride, the Lamb's wife. (10) And he carried me away in the spirit to a great and high mountain, and shewed me that great city, the holy Jerusalem, descending out of heaven from God, (11) Having the glory of God: and her light was like unto a stone most precious, even like a jasper stone, clear as crystal; (12) And had a wall great and high, and had twelve gates, and at the gates twelve angels, and names written thereon, which are the names of the twelve tribes of the children of Israel: (13) On the east three gates; on the north three gates; on the south three gates; and on the west three gates. (14) And the wall of the city had twelve foundations, and in them the names of the twelve apostles of the Lamb.
Rev 21:22-27 KJV And I saw no temple therein: for the Lord God Almighty and the Lamb are the temple of it. (23) And the city had no need of the sun, neither of the moon, to shine in it: for the glory of God did lighten it, and the Lamb is the light thereof. (24) And the nations of them which are saved shall walk in the light of it: and the kings of the earth do bring their glory and honour into it. (25) And the gates of it shall not be shut at all by day: for there shall be no night there. (26) And they shall bring the glory and honour of the nations into it. (27) And there shall in no wise enter into it any thing that defileth, neither whatsoever worketh abomination, or maketh a lie: but they which are written in the Lamb's book of life.
Also foretold by Isaiah:
Isa 60:10-12 KJV And the sons of strangers shall build up thy walls, and their kings shall minister unto thee: for in my wrath I smote thee, but in my favour have I had mercy on thee. (11) Therefore thy gates shall be open continually; they shall not be shut day nor night; that men may bring unto thee the forces of the Gentiles, and that their kings may be brought. (12) For the nation and kingdom that will not serve thee shall perish; yea, those nations shall be utterly wasted.
Isa 60:14-16 KJV The sons also of them that afflicted thee shall come bending unto thee; and all they that despised thee shall bow themselves down at the soles of thy feet; and they shall call thee, The city of the LORD, The Zion of the Holy One of Israel. (15) Whereas thou hast been forsaken and hated, so that no man went through thee, I will make thee an eternal excellency, a joy of many generations. (16) Thou shalt also suck the milk of the Gentiles, and shalt suck the breast of kings: and thou shalt know that I the LORD am thy Saviour and thy Redeemer, the mighty One of Jacob.