'Secret' in the Bible

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.

And he said to me, “Do not seal up the words of the prophecy of this book, for the time [of their fulfillment] is near.