From the Analytical Lexicon of the Greek New Testament...
μυστηριον, ου, το. mystery, secret; (1) as a religious technical term in the cults of the Greco- Roman world, a religious secret confided only to the initiated, secret rite, not used in the NT; (2) in the NT; (a) as what can be known only through revelation mediated from God what was not known before (MT 13. 11); (b) as a supreme redemptive revelation of God through the gospel of Christ mystery (RO 16. 25; EP 3. 9); (c) as the hidden meaning of a symbol with metaphorical significance mystery (EP 5. 32 (ANLEX)
Romans 11:25 For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in.
Romans 16:25-26 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,
But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith:
1 Corinthians 15:51-52 Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed,
In a moment, in the twinkling of an eye, at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed.
Ephesians 1:9-10 Having made known unto us the mystery of his will, according to his good pleasure which he hath purposed in himself:
That in the dispensation of the fulness of times he might gather together in one all things in Christ, both which are in heaven, and which are on earth; even in him:
Ephesians 3:3-6 How that by revelation he made known unto me the mystery; (as I wrote afore in few words,
Whereby, when ye read, ye may understand my knowledge in the mystery of Christ)
Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit;
That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:
Ephesians 3:9-10 And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ:
To the intent that now unto the principalities and powers in heavenly places might be known by the church the manifold wisdom of God,
Colossians 1:26-27 Even the mystery which hath been hid from ages and from generations, but now is made manifest to his saints:
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:
All sound like definition #2 to me.