D&C 107

The Lord taught -

"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, to commune with the general assembly and church of the Firstborn, and to enjoy the communion and presence of God the Father, and Jesus the mediator of the new covenant." (Doctrine and Covenants Section 107:18 - 19)

In these verses, I believe the Lord is teaching a duel meaning of keys of the Melchizedek priesthood. First are keys of administration held by presidents over priesthood organizations with the president of the Church holding all keys with authority to exercise all those keys of administration and all blessings in the Church. In this position holds the keys of revelation for the Church. The Prophet Joseph Smith taught -

"The Melchizedek High Priesthood was no other than the Priesthood of the Son of God; . . . there are certain ordinances which belong to the Priesthood, from which flow certain results; and the Presidents or Presidency are over the Church; and revelations of the mind and will of God to the Church, are to come through the Presidency. This is the order of heaven, and the power and privilege of this Priesthood." (History of the Church, 2:477.)

But keys also represent the tool to unlock. Abraham described his desire to hold the Melchizedek priesthood saying -

"...finding there was greater happiness and peace and rest for me, I sought for the blessings of the fathers, and the right whereunto I should be ordained to administer the same; having been myself a follower of righteousness, desiring also to be one who possessed great knowledge, and to be a greater follower of righteousness, and to possess a greater knowledge, and to be a father of many nations, a prince of peace, and desiring to receive instructions, and to keep the commandments of God, I became a rightful heir, a High Priest, holding the right belonging to the fathers." (Pearl of Great Price Abraham 1:2)

Abraham understood that he needed to receive this priesthood to "possess a greater knowledge". The same is true of the saints today. Receiving the priesthood is the key for -

"...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)

The Prophet Joseph Smith taught that the Melchizedek priesthood -

“...is the channel through which all knowledge, doctrine, the plan of salvation, and every important matter is revealed from heaven” (History of the Church, 4:207)

I believe this would be in reference to the Church as a whole and the individual. President Spencer W. Kimball also explained how the Melchizedek priesthood is the key to all spiritual blessings in the Church -

"It is the means whereby the Lord acts through men to save souls. Without this priesthood power, men are lost. Only through this power does man ‘hold the keys of all the spiritual blessings of the church,’ enabling him to receive ‘the mysteries of the kingdom of heaven, to have the heavens opened’ unto him enabling him to enter the new and everlasting covenant of marriage and to have his wife and children bound to him in an everlasting tie, enabling him to become a patriarch to his posterity forever, and enabling him to receive a fullness of the blessings of the Lord.” (“The Example of Abraham,” Ensign, June 1975, p. 3.)