What is the mystery of Christ?Eph. 3:3-11 'How that by revelation he made known unto me the mystery; (as I wrote afore in few words, Whereby, when ye read, ye may understand my knowledge in the mystery of Christ) Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit; That the Gentiles should be fellow heirs, and of the same body, and partakers of his promise in Christ by the gospel:Whereof I was made a minister, according to the gift of the grace of God given unto me by the effectual working of his power. Unto me, who am less than the least of all saints, is this grace given, that I should preach among the Gentiles the unsearchable riches of Christ; And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ: To the intent that now unto the principalities and powers in heavenly places might be known by the church the manifold wisdom of God, According to the eternal purpose which he purposed in Christ Jesus our Lord:'
Eph. 6:19 'And for me, that utterance may be given unto me, that I may open my mouth boldly, to make known the mystery of the gospel,"
Col 1:26 'Even the mystery which hath been hid from ages and from generations, but now is made manifest to his saints:'
In the above verses Paul tells us that "this mystery" that has been hid in God from the beginning of the world, is now made known to the Apostles and Prophets…."this mystery" is that the Gentiles are now made fellow heirs of promise of Christ, which is 'the Gospel' which means we all (Gentiles included) can now be saved and partake in 'Eternal Life'. If this 'mystery of Christ' is allowing the Gentiles to fully partake in Salvation along with the Jew, then the mystery that Paul is speaking of in Romans that he doesn’t want them to be ignorant of is also of that same promise….
…..that the Gentiles 'appointed time' has come, and when their fullness is come in (that is the going forth of the Gospel into all the world), then that defines 'the manner' in which all 'Israel' shall be saved….through Christ alone.Rom. 11:25-27 'For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness (pleroma= meaning to fill up, to finish) of the Gentiles be come in. And so (houto=in this manner) all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob: For this is my covenant unto them, when I shall take away their sins.'
Luke 21:24 'And they shall fall by the edge of the sword, and shall be led away captive into all nations: and Jerusalem shall be trodden down of the Gentiles, until the times (kairos=appointed time) of the Gentiles be fulfilled (pleroo=to make full, to complete).'
In tying all of the above verses together we first see the 'mystery of Christ' being defined as 'the Gospel', that is now made available for the Gentile, and before the 'end comes' that Gospel must be sent out and preached to 'all the nations'. This is what is said to be 'the fullness of the Gentiles' in Luke and Romans, so when that happens the end will come, and that is the way that all of Israel will be saved, no longer will it be by the Temple sacrifices for those are done away with…..now all who are saved must come through Christ.Matt. 24:14 'And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; (ethnos=Gentiles) and then shall the end come.'
Rev. 10:7 'But in the days of the voice of the seventh angel, when he shall begin to sound, the mystery of God should be finished, as he hath declared to his servants the prophets.'
Rev. 14:6-7 'And I saw another angel fly in the midst of heaven, having the everlasting gospel to preach unto them that dwell on the earth, and to every nation, and kindred, and tongue, and people, Saying with a loud voice, Fear God, and give glory to him; for the hour of his judgment is come: and worship him that made heaven, and earth, and the sea, and the fountains of waters.'
When the seventh angel in Revelation 10 sounds it is declared the 'mystery of God' should be finished, that is referring to the Gospel being preached to all the nations, which is what Paul and the other Apostles did. That is what the symbolic vision in Rev. 14 of the angel having the 'everlasting Gospel' to preach to 'every nation' before the 'hour of judgment' comes is.
Once judgment is poured out upon Jerusalem (that happened in 70 A.D.), and the temple is gone, and there are no more sacrifices- the only way to salvation is through Jesus Christ, that is the only way to be saved, which includes all of Israel…..they are no longer separate, all are one in Christ.