How plain! What we have in the gospel is a revelation. We must tie this concept of God's revelation to the word "mystery" (verse 7).
In English it does not mean exactly the same thing as in Greek. In English, mystery means "a puzzle that is difficult to solve," but in Greek, it means "a secret that is impossible to penetrate." So, the Word of God, His purpose and plan, is a mystery, a secret that is impossible to penetrate. Paul is implying that man would never find out what God intends, except that God gives it to us by revelation.
We have in no way earned this revelation. We have it because it pleased God to give it to us. He withholds it from others, but He has given it to us. He is in no way beholden to us, as if He owed us something. We could dig in His Word over our entire lifetimes and never come to what He freely gives to us for His purposes, for His own reasons.
Brilliant men like Adam Clarke have dug into God's Word through the centuries. It took him forty years to produce his famous commentary. Considering that the man was unconverted, it really is a magnificent work, done with all sincerity and dedication. Yet, at the end of his efforts, he did not fully penetrate the mystery of what God is doing among men. A brilliant man and a brilliant work, yet he emerged from his studies not understanding the divine purpose that God gives to us without our earning it. On the other hand, it is very likely that many of us never cracked the pages of a Bible before God began to open our minds. Some have, some have not. But God called many of us in that situation, and though we did not deserve it, He revealed His way to us.