The most powerful position under the king was that of the royal steward—the prime minister. King Solomon instituted the office in 1 Kings 4:6 when he appointed Ahi’shar (Ahi’shar was in charge of the palace), and Isaiah provides a more thorough description of the office. Now that you have re-read Matthew 16:19, please read what Jesus was referencing, and what Jesus’ audience would have recognized as the structural paradigm of the new kingdom—of the true Church of Christ:

And I will place on his [Eli’akim’s] shoulder the key of the house of David; he shall open, and none shall shut; and he shall shut, and none shall open (Isaiah 22:20-22).

Eli’akim was a type of St. Peter—his position was a type of the papacy. As shown in Isaiah, Eli’akim was given the key to the kingdom and was promoted to the most prestigious office in the kingdom—the master of the palace (Isaiah 22:15 New American Bible). Eli’akim’s position was that of prime minister; sharing in the king’s authority, governing in the king’s name, and acting for him in the king’s absence. Keys are a symbol of power, granting or denying admittance to the royal presence.

Matthew 16:19 shares a royal context provided by the Old Testament Scriptures, and supported by a societal context that acknowledged how kingdoms are governed. Jesus was saying that St. Peter would be the new master of the palace in His kingdom. Since the keys symbolized how the Davidic king vested his prime minister with his very authority, Jesus was vesting St. Peter with the keys to His kingdom, essentially: “You, Peter, will be the Prime Minister in my kingdom.”  

The office of prime minister is pre-supposed, and Jesus chose to utilize that pre-supposed hierarchical paradigm when He assigned the keys to St. Peter. This might be the first time you have seen this connection; can you ignore the similar language and parallel structure of the passages found in Matthew and Isaiah? Do you believe the earliest Christians (Jewish converts) would not have noticed? Did Jesus intend to create an autonomous ecclesial structure when he chose to utilize a hierarchical Davidic paradigm when he called St. Peter to office? St. Peter seems to have understood the steward would be set over Christ’s household, because Jesus asked him, Who then is the faithful and wise steward, whom his master will set over his household, to give them the portion of food at the proper time? (Luke 12:42, emphasis added).

However, many non-Catholic Christians prefer a more cryptic interpretation of Jesus’ words from Matthew 16—a preference to deny a practical existence of the Old Testament types; resulting in a theory that Jesus did not realize the confusion He would create for future Protestant sects by quoting Isaiah when calling St. Peter to office. But nobody in the historical Catholic Church of Christ was confused. Even the secular world throughout history has recognized that St. Peter and his office held authority over the worldwide Church; the theory of Church-wide ecclesial autonomy is no older than Protestantism itself, and the drive for personal promotion is as ancient as the fall of man.