As we study the Word of God rightly divided we are to understand that God has arranged His dealings with mankind into two programs. We have His prophesied purpose and His secret purpose. Prophecy has to do with the earth and Christ's reign upon it during the millennial kingdom, while the Mystery concerns our exaltation with Christ in the heavenlies.
There is no mystery, at least not the kind you want. In real life there are no fogbound moors or clues on matchbooks or fifth columnists waiting to be unmasked. it would be nice if here were, because then there would be solutions to things in life, but it doesn't always work that way. Everyone likes a good detective story. I went through my Hammett phase in college. I think the attraction is, in life our mysteries aren't exciting. You know They're just intractable and depressing and enervating. Like, why do we always hurt the ones we love. Where does the money go ...in a detective story, at least the universe makes sense. It was him. He did it. The natural order is disturbed, but the beauty of it is that it's restored again.
Paul's evangelistic mission was different from the Kingdom Apostles, but in many ways the same. The Kingdom Apostles preached Jesus the Messiah, ready to return to Israel and set up His Messianic Kingdom. From Israel, the Apostles, with Christ on David's Throne, were to go into all the world and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, and teaching them to observe everything Christ had commanded them. They would do that because all authority in heaven and on earth had been given to Christ. As we all know, that did not happen. It will happen in the future, but it has not yet happened. What has happened is that Christ revealed a Mystery through His Special Apostle Paul.