Paul’s message, was delivered to him by Jesus, on the road to Damascus, and unless you believe, he was revealing it unto the wise and prudent (the rest of the world) and that it wasn’t hidden from all, as it says specifically, you can see it was truly a mystery.
“Which in other ages was not made known” The dispensation of grace
Ephesians 3 King James Version (KJV)
3 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: