Updated: Mar 23
Christ granted Peter the authority necessary to function in his papal office, and He granted him keys to execute that authority. To understand this doctrine of Peter and the Keys, we have to go back to the Old Testament.
In various places in the Old Testament, such as 2 Sam. 7:16, Psalm 89:3-4, and 1 Chron.17:12,14, God solemnly promises or, as in Chronicles, covenantally vows, to institute a Kingdom, specifically His Kingdom, through David's line. This Kingdom would be one that would last forever as "he shall build a house for [God], and [God] will establish his throne forever" (1 Chron.17:12). Here, God swears a covenant oath to establish the Davidic Kingdom as one that would last forever.
We move forward to Matthew 1:1. The Evangelist Matthew takes great care and effort to establish that, in the New Testament, it is Jesus who is the true Davidic heir. The Evangelist Luke, in Luke 1:32 also clearly has the archangel Gabriel announcing to Mary that the Son she would bear would be given “the throne of His father David.”
It was common practice of the old Kings of Israel to appoint a Prime Minister who would stand in a place of preeminence over the other ministers of his cabinet. This Prime Minister would possess all of the authority necessary to administrate all affairs of the Kingdom in the place of the King. As such, the Prime Minister answers to the King Himself.
In confirming that Christ as the legitimate heir to David's throne, Matthew and Luke show that Christ is the new and eternal Davidic King. As such, to every Jew who lived in Jesus's time, it would come as no strange thing that Christ would establish, for His Kingdom, a Prime Minister. Such an action would have been deemed as commonplace as unleavened bread at Passover. Hence, Christ establishing Peter as His Rock and subsequently conferring Keys upon him in Matt 16:18-19 meant that Peter was now Christ's Prime Minister to His Kingdom, the Vicar to His Church, His chief steward to His house.
In Matt. 16:19 Jesus gives Peter the “keys of the kingdom of heaven.” One of the common protestant arguments against the legitimacy of this verse is that the Kingdom Christ is allegedly referring to is His eternal Kingdom, in Heaven. Nonetheless, the verse itself answers for us that the Keys Christ gives Peter are for BOTH Heaven and earth, for Christ tells Him, "whatsoever thou shalt bind on earth shall be bound in Heaven, and whatsoever thou shalt loose on earth shall be loosed in Heaven" (Matt 16:19).