I was looking up Bible references to “mystery” and found this excellent resource.
The Twelve Mysteries of Scripture, by Vine, 121 page book pdf written by William Edwy Vine (1873–1949). Public domain.
“THE word “mystery” in Scripture does not denote that which is mysterious. Its meaning therefore differs from that of its current use in English. In Scripture it denotes that which, being outside the range of unassisted natural apprehension, can be made known only by divine revelation, and is made known in a manner and at a time appointed by God, and to those only who are illumined by His Spirit.
“In the ordinary sense a mystery implies knowledge withheld; its
Scriptural significance is truth revealed. Hence the terms especially associated with the subject are “made known”, “manifested”, “revealed”, “preached”, “understand”, “dispensation”.
“The definition given above may be best illustrated by the following passages.
“The mystery which hath been hid from all ages and generations: but now hath it been manifested to His saints.” Colossians 1:26
“The mystery of Christ, . . . which in other generations was not made known unto the sons of men, as it hath now been revealed unto His holy Apostles and Prophets in the Spirit.” Ephesians 3:5
“The mystery which hath been kept in silence through times eternal, but now is manifested, and by the Scriptures of the Prophets,
according to the commandment of the eternal God, is made known unto all the nations unto the obedience of faith.” Romans 16:25.
Mystery from Vine’s Expository Dictionary. Link
“Primarily that which is known to the mustes, “the initiated” (from mueo, “to initiate into the mysteries;” cp. Philippians 4:12 , mueomai, “I have learned the secret,” RV). In the NT it denotes, not the mysterious (as with the Eng. word), but that which, being outside the range of unassisted natural apprehension, can be made known only by Divine revelation, and is made known in a manner and at a time appointed by God, and to those only who are illumined by His Spirit. In the ordinary sense a “mystery” implies knowledge withheld; its Scriptual significance is truth revealed. Hence the terms especially associated with the subject are “made known,” “manifested,” “revealed,” “preached,” “understand,” “dispensation.” The definition given above may be best illustrated by the following passage: “the mystery which hath been hid from all ages and generations: but now hath it been manifested to His saints” (Colossians 1:26 , RV). “It is used of:
“(a) spiritual truth generally, as revealed in the gospel, 1 Corinthians 13:2; 14:2 (cp. 1 Timothy 3:9 ). Among the ancient Greeks ‘the mysteries’ were religious rites and ceremonies practiced by secret societies into which any one who so desired might be received. Those who were initiated into these ‘mysteries’ became possessors of certain knowledge, which was not imparted to the uninitiated, and were called ‘the perfected,’ cp. 1 Corinthians 2:6-16 where the Apostle has these ‘mysteries’ in mind and presents the gospel in contrast thereto; here ‘the perfected’ are, of course, the believers, who alone can perceive the things revealed;
“(b) Christ, who is God Himself revealed under the conditions of human life, Colossians 2:2; 4:3, and submitting even to death, 1 Corinthians 2:1 (in some mss., for marturion, testimony),7, but raised from among the dead, 1 Timothy 3:16, that the will of God to c ordinate the universe in Him, and subject it to Him, might in due time be accomplished, Ephesians 1:9 (cp. Revelation 10:7), as is declared in the gospel, Romans 16:25; Ephesians 6:19;
“(c) the Church, which is Christ’s Body, i.e., the union of redeemed men with God in Christ, Ephesians 5:32 (cp. Colossians 1:27 );
“(d) the rapture into the presence of Christ of those members of the Church which is His Body who shall be alive on the earth at His Parousia, 1 Corinthians 15:51;
“(e) the operation of those hidden forces that either retard or accelerate the Kingdom of Heaven (i.e., of God), Matthew 13:11; Mark 4:11;
“(f) the cause of the present condition of Israel, Romans 11:25;
“(g) the spirit of disobedience to God, 2 Thessalonians 2:7; Revelation 17:5,7; cp. Ephesians 2:2 .” * [* From Notes on Thessalonians, by Hogg and Vine, pp. 256,257.]
To these may be added
“(h) the seven local churches, and their angels, seen in symbolism, Revelation 1:20;
“(i) the ways of God in grace, Ephesians 3:9. The word is used in a comprehensive way in 1 Corinthians 4:1.