The Apostle Paul writes:

"How that by revelation he (the ascended Christ) 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 (Paul, Barnabas, Silas, etc., not the twelve) and prophets (preachers) by the spirit; That the Gentiles should be fellowheirs, and of the same body (of Christ), and partakers of his promise in Christ by the gospel" (Eph.3:3-6).

While the world was due for the judgment of God after the stoning of Stephen in Acts 7, the Lord Jesus Christ in mercy and grace reached down from heaven to save "the chief of sinners" - Saul Paulus of Tarsus – then called Paul. By this God "shewed forth all longsuffering" to this hopeless world.

Unlike Jesus' twelve Apostles and their message of the "gospel of the kingdom" for Israel alone, Paul was made a minister of "the church, which is His body," to be the dispenser of God's grace message during today's "dispensation of the grace of God" (Eph 3:2).

The Apostle Paul wrote this.

"who now rejoice in my (Paul's) sufferings for you, and fill up that which is behind of the afflictions of Christ in my flesh for his body's sake, which is the church:

²⁵Whereof I am made a minister, according to the dispensation of God which is given to me for you, to fulfil (Gk. pleroo, to complete) the word of God;

²⁶Even (that is,) the mystery which hath been hid from ages and from generations, but now is made manifest to his saints:

²⁷To whom God would make known what is the riches of the glory of this mystery among the Gentiles; which is Christ in you, the hope of glory" (col.1:24-27).

Paul declares that those things concerning the church, "the body of Christ," were held in "secret" aforetime, "not made known" (Eph.3:5), "...hid in God..." (3:9) and, "hid from ages and from generations, but now is made manifest to his saints" (Col.1:26).

Therefore he is saying that "this mystery (secret plan of God)" is not found in the Old Testament scriptures. You may search the Old Testament and you will find that its absence is conspicuous. And furthermore, in Galatians, Paul tells us again exactly how he got received message:

"But I certify you, brethren, that the gospel which was preached of me is not after man. ¹²For I neither received it of man, neither was I taught it, but (rather) by the revelation of (from) Jesus Christ" (Gal.1:11-12).

Paul "received" his gospel directly from the Lord Jesus Christ. No man taught Paul the gospel that he preached. He "received" it "by the revelation of Jesus Christ."

Paul boldly proclaims that he did not preach Jesus Christ according to a prophetic gospel passed on by men, referred to in Matthew, Mark, Luke, & John.

Paul was specially "chosen" of God; and through Paul "this mystery among the gentiles" was to be revealed to the world. The message that Paul preached in his thirteen epistles concerning the revelation of Jesus Christ was according to the unveiling of "the mystery," as summarized most succinctly in Paul's words here.

Romans 16:25-26 (Darby) Now to him that is able to establish you, according to my glad tidings and the preaching of Jesus Christ, according to [the] revelation of [the] mystery, as to which silence has been kept in [the] times of the ages.