St. Paul took it as his commission that he was to reveal the mysterion of Christ to the gentiles, and we can see that he was here using a language that would have been somewhat familiar to ‘the pagans.’
I am made a minister according to the dispensation of God, which is given me towards you, that I may fulfill the word of God: the mysterion which hath been hidden from ages and generations, but now is manifested to his saints, to whom God would make known the riches of the glory of this mysterion among the Gentiles, which is Christ in you, the hope of glory.
He mentions over twenty times ‘the mystery made known to me by revelation’:
According to revelation, the mystery has been made known to me, as I have written above in a few words: As you reading, may understand my knowledge in the mystery of Christ, which in other generations was not known to the sons of men, as it is now revealed to his holy apostles and prophets in the Spirit: That the Gentiles should be fellow heirs and of the same body: and copartners of his promise in Christ Jesus, by the gospel of which I am made a minister, according to the gift of the grace of God, which is given to me according to the operation of his power. To me, the least of all the saints, is given this grace, to preach among the Gentiles the unsearchable riches of Christ: and to enlighten all men, that they may see what is the dispensation of the mystery which hath been hidden from eternity in God who created all things.
We can see here that mysterion refers to a sacred reality, and not to something that remains veiled or hidden or secret, although it can be all of those things depending on one’s comprehension: but because St. Paul is fairly clear in that the mystery has been revealed to him, resulting in a real knowledge of it, that it is not something entirely incomprehensible but is essential to the Christian revelation.
Howbeit we speak wisdom among the perfect: yet not the wisdom of this world, neither of the princes of this world that come to nought. But we speak the wisdom of God in a mystery, a wisdom which is hidden, which God ordained before the world, unto our glory: Which none of the princes of this world knew. For if they had known it, they would never have crucified the Lord of glory. But, as it is written: That eye hath not seen, nor ear heard: neither hath it entered into the heart of man, what things God hath prepared for them that love him. But to us God hath revealed them by his Spirit. For the Spirit searcheth all things, yea, the deep things of God.
The sacred reality—the mysterion—has been made known: and this knowledge forms the core of St. Paul’s message.
 Col. 1:25-27.
 Eph. 3:3.
 Eph. 3:3-9.
 1 Cor. 2:6-10.