God is the keeper of mysteries, and also Jesus Christ. He reveals them to those with a pure conscience. There are two main mysteries: the mystery of faith and the mystery of wickedness.
Faith
1Ti 3:16
And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory.
Rev 10:7
But in the days of the voice of the seventh angel, when he shall begin to sound, the mystery of God should be finished, as he hath declared to his servants the prophets.
1Ti 3:9
Holding the mystery of the faith in a pure conscience.
Wickedness
2Th 2:6,7
And now ye know what withholdeth that [the man of sin] might be revealed in his time. For the mystery of iniquity doth already work: only he who now letteth
will let, until he be taken out of the way. And then shall that Wicked be revealed, whom the Lord shall consume with the spirit of his mouth, and shall destroy with the brightness of his coming:
Even him, whose coming is after the working of Satan with all power and signs and lying wonders, and with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved.
And for this cause God shall send them strong delusion, that they should believe a lie: that they all might be damned who believed not the truth, but had pleasure in unrighteousness