'Secret' in the Bible

but upholding and fully understanding the mystery [that is, the true doctrine] of the [Christian] faith with a clear conscience [resulting from behavior consistent with spiritual maturity].

And great, we confess, is the mystery [the hidden truth] of godliness:He (Jesus Christ) who was revealed in human flesh,Was justified and vindicated in the Spirit,Seen by angels,Preached among the nations,Believed on in the world,Taken up in glory.