Paul’s mystery gospel is revealed in the New Testament revelation. The mystified mind of the reader disables the reader from comprehending the revelation. The mystified mind can only be demystified by the power of God the Holy Spirit.
God the Holy Spirit is the resident teacher inside the in Christ grace believer Saint. He child trains us to become demystified. He illuminates our hearts to the truth in His word.
There’s not a clear definition of Paul’s Mystery Gospel in any single verse. It takes lots of work on our part in Bible study to pick and sift through Pauls epistles for every mention of it. Paul’s Mystery Gospel is not addressed to Carnal grace believing Saints in Christ, but only to the mature.