'Hidden' 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.

Likewise, good works are obvious, and those that are not obvious cannot remain hidden.