I'm speaking of mystery meaning what is not known. It is something that only a few understand, or know. For example, an artist and his creation. The artist is trying to convey something with his work. We viewing it may understand some of it, but now all the artist's intention. This is how "mystery" is meant, in the Christian sense.
The word "mystery," itself, is from Greek. It's meaning is, not something obscure, but a secret given only to a few. Did you know that "mystery" occurs 28 times in the New Testament? The word is used chiefly by Paul, who knew Greek. Paul accepted this term (mystery) to indicate the fact that, his Gospel message had been revealed to him by the Christ.
The word in a few passages refers to a symbol, allegory or parable, which conceals its meaning from those who look only at the literal sense, but is the medium of revelation to those who have been given the grace to understand. Its most common meaning is that of a divine truth once hidden, but now revealed in the Gospel.
The Greek is, "musterion." A mystery, as in something into which one must be initiated, instructed, before it can be known, something of itself not obvious and above human insight.
And the Christian references:
Generally. Mark 4:11. Luke 8:10. Matt. 13:11, Jesus said, "It is given unto you to know the mysteries = (hidden truths) of the kingdom of heaven," as in a deeper and more perfect manner than they were made known to others.
1 Cor. 14:2, "For he that speaketh in an unknown tongue (language) speaketh not unto men, but unto God: for no man understandeth him, howbeit in the spirit he speaketh mysteries."
Eph. 5:32, "This is a great mystery: but I speak concerning Christ and the church."
2 Thes. 2:7, "the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way," mysterious wickedness, as hidden, as yet unknown to Christians, opposed to, ver.8, "And then shall that Wicked be revealed."
Rev.1:20, "The mystery of the seven stars which thou sawest in my right hand --- are the angels (messengers) of the seven churches."
Rev.10:7, "in the days of the voice of the seventh angel, when he shall begin to sound, the mystery of God should be finished."
Rev. 17:5, "upon her forehead was a name written, mystery Babylon the great, the mother of harlots and abomination of the earth," with ver.7, "the angel said, --- I will tell thee the mystery of the woman, and of the beast."
Eph.3:9, "to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God," and
Col.1:26, "even the mystery which hath been hid from ages and from generations, but now is made manifest to his saints."
Eph. 6:19, "for me, that utterance may be given unto me, that I may open my mouth boldly, to make known the mystery of the gospel."
Col. 2:2, "their hearts might be comforted, being knit together in love, and unto all riches of the full assurance of understanding, to the acknowledgement (knowledge) of the mystery of God."
Col. 4:3 "pray that God would open unto us a door of utterance, to speak the mystery of Christ."
Eph. 3:4, "When ye read, ye may understand my knowledge in the mystery of Christ."
1 Tim.3:9, "Holding the mystery of the faith in pure conscience."
Rom.16:25, "the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began."
1 Cor.2:7, 4:1, "Let a man so account of us (consider us,) as of the ministers of Christ, and stewards of the mysteries of God."
1 Cor.13:2, "though I have the gift of prophecy and understanding all mysteries, --- so that I could remove mountains, and have not charity, I am nothing."
Eph.3:3, "How that by revelation he made known unto me the mystery; (as I wrote afore in few words."
Col.1:27, "To whom God would make known what is the riches of the glory of this mystery among the Gentiles; which is Christ in you, the hope of glory"
Rom.11:25, "For I would not, brethren, that ye should be ignorant of this mystery, --- that blindness in part is happened to Israel, until the fulness of the Gentiles be come in."
1 Cor.15:51, "behold, I shew you a mystery; We shall not all sleep, but we shall all be changed."
Eph.1:9, "Having made known unto us the mystery of His will, according to His good pleasure which He hath purposed in Himself."
1 Tim. 3:16, "great is the mystery of godliness, God = (Jesus) was manifest in the flesh, justified in the Spirit."
h/t to Brother Peter Totleben, O.P., and the Seek First Wisdom site.