D&C 76

We learn in the revelation of the priesthood that -

"...this greater priesthood administereth the gospel and holdeth the key of the mysteries of the kingdom, even the key of the knowledge of God."  (Doctrine and Covenants | Section 84:19)

This doctrine is taught again in a later revelation -

"The power and authority of the higher, or Melchizedek Priesthood, is to hold the keys of all the spiritual blessings of the church—
19 To have the privilege of receiving the mysteries of the kingdom of heaven, to have the heavens opened unto them..."  (Doctrine and Covenants | Section 107:18 - 19)

This promise and blessing is according to the Oath and Covenant of the priesthood where -

"...whoso is faithful unto the obtaining these two priesthoods of which I have spoken, and the magnifying their calling..."  (Doctrine and Covenants | Section 84:33)

This oath and covenant is not only to those that obtain these two priesthoods, but -

"...also all they who receive this priesthood receive me, saith the Lord;
36 For he that receiveth my servants receiveth me..."  (Doctrine and Covenants | Section 84:35 - 36)

Which would include all men and women that receive the doctrine and ordinances administered by the priesthood.  Of these the promise is -

"...all that my Father hath shall be given unto him."  (Doctrine and Covenants | Section 84:38)

Considering that the priesthood is the "key of the knowledge of God", then, in my mind, the the promise of receiving all that the Father hath would suggest to me, all His knowledge.  As the Prophet Joseph and Sidney Rigdon were in the Spirit, the Lord said -

"...I, the Lord, am merciful and gracious unto those who fear me, and delight to honor those who serve me in righteousness and in truth unto the end.
6 Great shall be their reward and eternal shall be their glory."  (Doctrine and Covenants | Section 76:5 - 6)

Obviously, these would be those the receive His priesthood and magnify their calling or serve Him.  Of such, He again teaches -

"...to them will I reveal all mysteries, yea, all the hidden mysteries of my kingdom from days of old, and for ages to come, will I make known unto them the good pleasure of my will concerning all things pertaining to my kingdom.
8 Yea, even the wonders of eternity shall they know, and things to come will I show them, even the things of many generations.
9 And their wisdom shall be great, and their understanding reach to heaven; and before them the wisdom of the wise shall perish, and the understanding of the prudent shall come to naught.
10 For by my Spirit will I enlighten them, and by my power will I make known unto them the secrets of my will—yea, even those things which eye has not seen, nor ear heard, nor yet entered into the heart of man."  (Doctrine and Covenants | Section 76:7 - 10)

I will emphasize that the mysteries of God is knowledge that only comes by revelation.  The Bible Dictionary defines mystery as -

"...a spiritual truth that was once hidden but now is revealed, and that, without special revelation, would have remained unknown."  (Bible Dictionary | M Mystery:Entry)

Mysteries would then be the knowledge of God given only by revelation.