D&C 107

On the topic of the priesthood, the Lord teaches -

"The power and authority of the higher, or Melchizedek Priesthood, is to hold the keys of all the spiritual blessings of the church—"  (Doctrine and Covenants | Section 107:18)

In the early days of modern revelation, many words were used interchangebly with different meanings just as the dictionary can provide different meanings for the same word.  Some of these words were - keys, ordain, etc.  As the Church has grown, there was more of an effort to use different words as to not confuse the meaning with the members.  For example, the word "ordain" now is used more in reference with priesthood ordinations where "set apart" is in reference to callings even though in the early days of the Church they were used interchangebly.

I believe the same is in regards to the word "keys" where today we reference priesthood keys as the right of a president to direct, control, and govern.  But in the case in this revelation, I believe the use of the word "keys" is in reference to the authority and privilege to unlock doors to ministering spiritual blessings, which would include when authorized, higher priesthood ordinances of the Melchizedek priesthood.  I believe that because of the words "is to hold".  Those that hold such authority and power to give priesthood blessings, and are to teach and be a example (Alma 13).  These are key to spiritual blessings.

The Lord then continues regarding these keys which are the keys of knowledge saying -

"To have the privilege of receiving the mysteries of the kingdom of heaven, to have the heavens opened unto them..."  (Doctrine and Covenants | Section 107:19)

Is this not a great spiritual blessing?

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)

Which suggests that we have the privilege to learn and come to understand truth and doctrine prophetic (scriptures) and personal revelation by the Holy Ghost.  I believe the scriptures are full of mysteries that as we study and keep His commandments, truths are revealed and understood.