'Mystery' 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.

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.

And on her forehead a name was written, a mystery: “BABYLON THE GREAT, THE MOTHER OF PROSTITUTES (false religions, heresies) AND OF THE ABOMINATIONS OF THE EARTH.”

But the angel said to me, “Why do you wonder? I will explain to you the mystery of the woman and of the beast that carries her, which has the seven heads and ten horns.