D&C 6

 The Lord said to the Prophet Joseph Smith and Oliver Cowdery - 

"Seek not for riches but for wisdom, and behold, the mysteries of God shall be unfolded unto you..."  Doctrine and Covenants 6:7)

Concerning the "mysteries of God", Joseph Fielding Smith taught - 

“The Lord has promised to reveal his mysteries to those who serve him in faithfulness.…There are no mysteries pertaining to the Gospel, only as we, in our weakness, fail to comprehend Gospel truth.…The ‘simple’ principles of the Gospel, such as baptism, the atonement, are mysteries to those who do not have the guidance of the Spirit of the Lord.” (Church History and Modern Revelation, 1:43.)

He told them - 

"Say nothing but repentance unto this generation..."  (Doctrine and Covenants 6:9)

I believe that means nothing but doctrine, principles, revealed truths, ordinances and covenants that lead to repentance.