Paul mentions the idea of mystery several times in his writings in scripture.
Eph 3:3 that by revelation there was made known to me the mystery, as I wrote before in brief. By referring to this, when you read you can understand my insight into the mystery of Christ.
From my readings in almost 100% of those instances he refers to the Gospel message of Jesus Christ as he uses the word mystery! Why do you think he calls this a mystery?
I believe, the reason he calls it a mystery is even though man may understand the gospel intellectually we do not understand it at a deeper level. When trials come to us we move back into old methods of how we relate to God and other people because the legalistic method is our natural tendency! Like Peter we quit acting in line with the truth of the Gospel (Gal 2:14)!
Later in Ephesians Paul mentions the one-flesh union between man and his wife as a mystery. He also relates Christ and the Church as a parallel to the one-flesh union. I believe the intimacy of a one-flesh emotional, spiritual, and physical intimacy is designed by God to be a re-enactment of the gospel of grace between a man and a woman in marriage. That is the mysterious one-flesh response between a man and his wife and its the mystery of the Gospel, the good news of Jesus death cross to proclaim victory over Satan and to rescue man from his sin.
Thats my only thought today.