The Mystery of Christ !
Some and if not most in false religion would have us believe that the Mystery of Christ in the Gospel was only revealed and known to Paul, but thats a Mistake, For God revealed it to all the Holy Apostles of Jesus Christ Eph 3:1-6
For this cause I Paul, the prisoner of Jesus Christ for you Gentiles,
2 If ye have heard of the dispensation of the grace of God which is given me to you-ward:
3 How that by revelation he made known unto me the mystery; (as I wrote afore in few words,
4 Whereby, when ye read, ye may understand my knowledge in the mystery of Christ)
5 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;
6 That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:
Lets look again at Vs 5, In the original its the Holy Apostles and Prophets, signifying the Particular Apostles of Christ, Pick by Him and equipped by Him !
This Mystery had to be revealed to the Apostles in order to equip them as Ministers of the New Testament/ Covenant 2 Cor 3:5-6
5 Not that we are sufficient of ourselves to think any thing as of ourselves; but our sufficiency is of God;
6 Who also hath made us able ministers of the new testament; not of the letter, but of the spirit: for the letter killeth, but the spirit giveth life.
Their sufficiency was from Christ the Head to be His New Covenant Ambassadors to Spiritual Israel, the Church, His Body Jn 15:5
5 I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing.
The Prophets of the OT which Preached Christ as He was to come, they were just as much the Body of Christ as was the Apostles/Prophets of the NT, for both looked to the foundation stone of Christ 1 Cor 3:11
11 For other foundation can no man lay than that is laid, which is Jesus Christ.
Christ is the foundation to the OT Prophets Isa 28:16
Therefore thus saith the Lord God, Behold, I lay in Zion for a foundation a stone, a tried stone, a precious corner stone, a sure foundation: he that believeth shall not make haste.
Therefore Paul and Isaiah believed on the same foundation stone Christ !
Isaiah , Jeremiah, Daniel and Ezekiel, their Hopes lied in Christ's Righteousness Imputed, for example Isa 54:17
17 No weapon that is formed against thee shall prosper; and every tongue that shall rise against thee in judgment thou shalt condemn. This is the heritage or inheritance of the servants of the Lord, and their righteousness is of me, saith the Lord.
16 In those days shall Judah be saved, and Jerusalem shall dwell safely: and this is the name wherewith she shall be called, The Lord our righteousness.
Dan 9:24
24 Seventy weeks are determined upon thy people and upon thy holy city, to finish the transgression, and to make an end of sins, and to make reconciliation for iniquity, and to bring in everlasting righteousness, and to seal up the vision and prophecy, and to anoint the most Holy.
Ezek 14:14,20
14 Though these three men, Noah, Daniel, and Job, were in it, they should deliver but their own souls by their righteousness, saith the Lord God.
20 Though Noah, Daniel, and Job were in it, as I live, saith the Lord God, they shall deliver neither son nor daughter; they shall but deliver their own souls by their righteousness.
This expression "by their righteousness" does not mean any personal righteousness of their own, but that Righteousness as spoken of by Isaiah, which is the inheritance unto the Servants of the Lord Isa 54:17, its the Righteousness of Faith, like said of Noah Heb 11:7
7 By faith Noah, being warned of God of things not seen as yet, moved with fear, prepared an ark to the saving of his house; by the which he condemned the world, and became heir of the righteousness which is by faith.
That same Righteousness Paul believed in Phil 3:9
9 And be found in him, not having mine own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith:
Paul as Noah was a Preacher of Righteousness 2 Pet 2:5
5 And spared not the old world, but saved Noah the eighth person, a preacher of righteousness, bringing in the flood upon the world of the ungodly;
Note: Its stated among the jews that Noah was a Prophet and Preacher ! Noah was a Prophet / Preacher in the midst of the world of the ungodly again 2 Pet 2:5 !
Also these things evidence the cohesion /continuity of the Church, the Body of Christ, from OT into the NT !