The definition of mystery is “a hidden or secret thing, not obvious to the understanding.”
Definition of musterion from Brown, Driver, Briggs Hebrew Lexicon:
“the secret counsels which govern God in dealing with the righteous, which are hidden from ungodly and wicked men but plain to the godly 3) in rabbinic writings, it denotes the mystic or hidden sense: of an OT saying, of an image or form seen in a vision, of a dream.“
From the time of the early Church Fathers, the word in Greek for mystery, musterion, took on the idea that it was something which could not be understood. This has carried through to our day and time, when we use the word mystery. It has the connotation that it cannot be known. But that is not true. As in the definition from BDB above, it is usually hidden from ungodly men, but “plain to the godly.” The rabbinic writings were accurate when they described a mystery as the hidden sense of something. It could be revealed by a vision, dream, or any method, but it is always a revealing of God. This is a good example of needing to understand a word from its biblical context.
Daniel 2:22 He revealeth the deep and secret things: he knoweth what is in the darkness, and the light dwelleth with him.
The word in Daniel for mystery is the same Aramaic word for mystery in the New Testament.
Regarding speaking in tongues,
I 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.
We do not understand what we are speaking in tongues to God, but in the process of speaking these “mysteries” with God, God reveals the hidden depth of things, situations and people around us. He even reveals the hidden things of our own heart.
1 Corinthians 2:7-10 But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory: 8 Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory. 9 But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him. 10 But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God.
These verses in I Corinthians refer to the great mystery of the body of Christ, how the Jews and Gentiles would be one “new man.” This was not revealed until after the day of Pentecost. But God knows many “hidden” things and he reveals them to those who seek for the deep things. I want to know those DEEP THINGS!
Spiritually, the hidden things are revealed. Once they are revealed they “belong to us and our children forever.” Deuteronomy 29:29 The secret things belong unto the LORD our God: but those things which are revealed belong unto us and to our children for ever, that we may do all the words of this law.
Many times, being quiet and speaking in tongues, something will come to mind to unfold and bring to light what to do or the understanding of a situation. That is what happened to Daniel when he sought for the interpretation of the dream for Nebuchadnezzar. That is what can happen to us when we pray for the unraveling of the hidden truths behind the scenes. God desires to reveal to us the hidden understandings, but it takes seeking him with our heart and being willing to hear. The greatest truths of the Word of God are not lying on the surface. They are unfolded to those who seek for them as for hidden treasures.
Proverbs 2:1-6 My son, if thou wilt receive my words, and hide my commandments with thee; 2 So that thou incline thine ear unto wisdom, and apply thine heart to understanding; 3 Yea, if thou criest after knowledge, and liftest up thy voice for understanding; 4 If thou seekest her as silver, and searchest for her as for hid treasures; 5 Then shalt thou understand the fear of the LORD, and find the knowledge of God. 6 For the LORD giveth wisdom: out of his mouth cometh knowledge and understanding.