'Mystic' in the Bible

[For my hope is] that their hearts may be encouraged as they are knit together in [unselfish] love, so that they may have all the riches that come from the full assurance of understanding [the joy of salvation], resulting in a true [and more intimate] knowledge of the mystery of God, that is, Christ,

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.