There is indeed a Great Mystery that the Apostle Paul reveals in his epistles. God chose Paul to write about this mystery because Paul's ministry was to the Gentiles but his background was Jewish. Being aware of both cultures enabled Paul to grasp what God told him to write. Most others could not handle the material and would have fought against it or even rejected it as profane. Because of Paul's meekness, he was able to receive the things that God had kept secret since the beginning of time.
25Now to him that is able to establish you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery which hath been kept in silence through times eternal,
26but now is manifested, and by the scriptures of the prophets, according to the commandment of the eternal God, is made known unto all the nations unto obedience of faith:
27to the only wise God, through Jesus Christ, to whom be the glory for ever. Amen.
Romans 16:25-27 American Standard Version (ASV)
We learn in these verses that God is able to establish His people through the gospel Paul taught, which was the preaching of Jesus Christ. In other words; Paul's gospel was the preaching (proclamation) of Jesus Christ. This gospel of Jesus Christ was in accordance with the revelation of the mystery which had kept in silence through times eternal.
These three verses tend to be very awkward as they are written. The translation of these verses in Conybeare and Howson is much more clear and easier to understand:
Now I commend you unto Him who is able to keep you steadfast, according to my Glad-tidings, and the proclamation of Jesus Christ - whereby is unveiled the mystery which was kept secret in eternal times of old;
But has now been brought to light, and made known to all the Gentiles by the scriptures of the Prophets, by command of the eternal God; that the Gentiles might be led to the obedience of faith - unto Him, the only wide God, I commend you through Jesus Christ; to whom be glory forever. Amen.
Conybeare and Howson - The Life and Epistles of St. Paul
Obviously this Great Mystery, which God kept secret in eternal times of old, has something to do with the Gentiles being able to be steadfast and obedient in the faith of Jesus Christ. The Prophets could see that something like this was possibly coming, but since they could not understand it; they rejected the idea in its entirety. What is spectacularly evident here is that the proclamation of Jesus Christ unveils this mystery which God kept secret since time began. This mystery has now been brought to light and made known unto all the Gentiles by the very command of God; which is that the Gentiles be led to the obedience of faith unto God through Jesus Christ.
How significant is this mystery? Without it, all of us who come from Gentile backgrounds would never hear, let alone be given the opportunity to believe the proclamation of Christ. Paul's insistence at preaching the good news of Jesus Christ to the Gentiles aroused the ire of the Jews toward him. It was because of Paul's ministry to the Gentiles that the Jews followed him, harassed him, disgraced him and even stoned him. In their minds, Paul was a traitor and was worthy of death. Such were the times in which Paul lived.
Because the good news of the Gospel of Jesus Christ was presented to the Gentiles, the Word of God was able to reach out to the non-Jewish world. In due time, the Gentile church surpassed the Jewish church (which is what they were afraid of) and through Paul, they mystery was revealed and presented for anyone and everyone to hear. As we progress in this study of the Great Mystery, we will see what in particular Paul taught that was so controversial and at the same time so MAGNIFICENT.