From mustees, "one initiated" into "a revealed secret"; mueoo the verb means "to conceal"; Mu (? ), the sound made by closing the lips (m), is the same onomatopoeic sound as in mute. In New Testament usage a spiritual truth heretofore hidden, incapable of discovery by mere reason, but now revealed. Not like the pagan mysteries, imparted only; to the initiated few. All Christians are the initiated; unbelievers alone are the uninitiated (2 Corinthians 4:3). The union of Christ and the church is such "a great mystery" (Ephesians 5:31-32). The church becoming a harlot by conformity to the world is a counter "mystery" (Revelation 17:5). "Iniquity" (anomia) in the harlot is a leaven working in "mystery" at first, i.e. latently; afterward when she is destroyed iniquity shall be revealed in "the man of iniquity" (ho anomos), the open embodiment of all previous evil, for popery cannot at once be the mystery of iniquity and the revealed antichrist (2 Thessalonians 2:7-8).
"The mystery of God" (Revelation 10:7), in contrast, is man's "redemption from all iniquity" and its consequences; a mystery once hidden in God's secret counsels, dimly-shadowed forth in types and prophecies, but now more and more clearly revealed according as the gospel kingdom develops itself up to its fullest consummation. "The mystery of godliness" (1 Timothy 3:16) is the divine scheme embodied in Christ (Colossians 1:26-27). Hidden before "with God" as the "mystery," He is now made manifest (John 1:1; John 1:14; Romans 16:25-26). Redemption for the whole Gentile world as well as Israel, to whom it seemed in a great measure restricted in Old Testament, is now revealed to all. "The glory of this mystery is Christ in you (now by faith as your hidden life, Colossians 3:8), the hope of glory" (your hereafter to be manifested life: 1 Corinthians 2:7-9; 2 Corinthians 4:17). There are six New Testament "mysteries":
(1) The incarnation (1 Timothy 3:16).
(2) The mystery of iniquity (2 Thessalonians 2:7).
(3) Christ's marriage to the church, Ephesians 5:32, translated "this mystery is great," i.e. this truth hidden once but now revealed, namely, Christ's spiritual union with the church, mystically represented by marriage, is of great import; not as Vulgate "this is a great sacrament"; not marriage in general, but that of Christ and His church, is the mystery, as Paul declares "I say it in regard to (eis) Christ, and to (eis)) the church," whereas Genesis 2:24 refers primarily to literal marriage. (See MARRIAGE.)
(4) The union of Jews and Gentiles in one body, the present election church (Ephesians 3:4-6); the Old Testament did not foretell we should form Christ's one body, the temple of the Holy Spirit; the Holy Spirit not merely gives influences as in Old Testament, but personally comes and dwells in the church, joining Jews and Gentiles in one fellowship of God and Christ; He is the earnest of the coming inheritance and the seal of redemption; the Old Testament saints had "proetermission" (paresis) of sins, the New Testament saints have "full remission" (afesis); the forbearance of God was exercised then, the righteousness of God is revealed now (Romans 3:25-26) in our justification.
(5) Israel's full and final restoration (Romans 11:25).
(6) The resurrection of the body (1 Corinthians 15:51).
Ordinarily "mystery" refers to those from whom the knowledge is withheld; in the New Testament mystery refers to those to whom it is revealed. It is hidden in God until brought forward; even when brought forward it remains hidden from the carnal. "Mysteries" (1 Corinthians 14:2) mean what is unintelligible to the hearers, exciting wonder rather than instructing; this is in the common sense, but the New Testament does not sanction in the gospel mysteries in this sense. In Revelation 1:20 "the mystery of the seven stars" is a oncehidden truth, veiled under this symbol, but now revealed; its correlative is revelation. In 1 Corinthians 13:2 "mysteries" refer to God's deep counsels heretofore secret but now revealed, "knowledge" to truths long known.
So in Matthew 13:11; Mark 4:11; Luke 8:10, "mysteries" answer in parallelism to "parables"; to the receptive "the mysteries," or once hidden things of the kingdom of God, are now known by God's gift; to the unbelieving they remain "parables," on which they see only the outward shell but do not taste the kernel (1 Corinthians 2:9-10; 1 Corinthians 2:14-15; Psalm 25:14; 1 John 4:20; 1 John 4:27; John 15:15). The parabolic form is designed to rouse the carnal to search and reflection; from whence Jesus did not begin to use it until after He had for some time been speaking plainly. In contrast to paganism, there were no mysteries revealed by God to ministers or priests that were not designed for all. Deuteronomy 29:29; "secret things belong to Jehovah (compare Job 11:7; Romans 11:33-34; at this point we must not presume to speculate; Colossians 2:18), 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."
The little ones must hear all revelation as much as the intellectual (Deuteronomy 6:7; Joshua 8:34-35; Nehemiah 8:1-2). Moses and the prophets and the apostles practiced no "reserve." So Jesus ordered (Matthew 10:27; Matthew 28:19). Paul preached publicly and from house to house the "whole counsel of God" (Acts 20:20; Acts 20:27), "keeping back nothing profitable." They taught babes indeed elementary essentials first, yet did not reserve the deepest truths out of sight, as the pagan mysteries; but set the ultimate goal of perfect knowledge from the first as that to be striven toward (1 Corinthians 2:6; 1 Corinthians 3:2; Hebrews 5:12).
Gnosticism introduced the system of esoteric and exoteric doctrine; the mediaeval church perpetuated it. Christ as God had the power to reserve His manifestation of Himself to a few during His earthly ministry, previous to the Pentecostal effusion of the Spirit (Mark 4:33; Mark 9:9; Luke 9:21); but His ministers have no such right. Paul disclaims it, 2 Corinthians 4:2; "we have renounced the hidden things of dishonesty, nor handling the word of God deceitfully, but by manifestation of the truth commending ourselves to every man's conscience in the sight of God." On men themselves rests the responsibility how they use the whole counsel of God set before them (2 Corinthians 2:15-16).
Fausset, Andrew Robert M.A., D.D., "Definition for 'mystery' Fausset's Bible Dictionary".