1 Corinthians 14:2
"For he that speaketh in an unknown tongue speaketh not unto men, but unto God: for no man understandeth him; howbeit in the spirit he speaketh mysteries."
Cross References (ranked by relevance)
And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues;
And when Paul had laid his hands upon them, the Holy Ghost came on them; and they spake with tongues, and prophesied.
For they heard them speak with tongues, and magnify God. Then answered Peter,
And though I have the gift of prophecy, and understand all mysteries, and all knowledge; and though I have all faith, so that I could remove mountains, and have not charity, I am nothing.
But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God.
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.
And for me, that utterance may be given unto me, that I may open my mouth boldly, to make known the mystery of the gospel,
That their hearts might be comforted, being knit together in love, and unto all riches of the full assurance of understanding, to the acknowledgement of the mystery of God, and of the Father, and of Christ;
But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory:
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.
Holding the mystery of the faith in a pure conscience.
Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed,
And they knew not that Joseph understood them; for he spake unto them by an interpreter.
And he said unto them, Unto you it is given to know the mystery of the kingdom of God: but unto them that are without, all these things are done in parables:
The LORD shall bring a nation against thee from far, from the end of the earth, as swift as the eagle flieth; a nation whose tongue thou shalt not understand;
And they that were with me saw indeed the light, and were afraid; but they heard not the voice of him that spake to me.
I will open my mouth in a parable: I will utter dark sayings of old:
Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began,
Go to, let us go down, and there confound their language, that they may not understand one another’s speech.
He answered and said unto them, Because it is given unto you to know the mysteries of the kingdom of heaven, but to them it is not given.