The Mystery of Godliness
What is a mystery?
Esoteric knowledge or practice, only for the specially initiated, a few; particularly religious mysteries in the cults of the 1st centuries. “Secret knowledge”
Gospel mysteries are things that were not known/have been hidden because of human limitation and must be revealed by God in order to be known. This mystery has been revealed by God now!
This 6-part statement that summarized the mystery of godliness was likely a hymn, creed, or saying of the time that Paul endorses here.