'Hidden' in the Bible

As for the mystery of the seven stars which you saw in My right hand, and the seven golden lampstands: the seven stars are the angels (divine messengers) of the seven churches, and the seven lampstands are the seven churches.

He who has an ear, let him hear and heed what the Spirit says to the churches. To him who overcomes [the world through believing that Jesus is the Son of God], to him I will give [the privilege of eating] some of the hidden manna, and I will give him a white stone with a new name engraved on the stone which no one knows except the one who receives it.’

but when it is time for the trumpet call of the seventh angel, when he is about to sound, then the mystery of God [that is, His hidden purpose and plan] is finished, as He announced the gospel to His servants the prophets.