In terms of Catholic preachability, todays Readings are a soft-ball pitch, a long high arc that every homilist ought to be able to knock out of the park. The lectionary readings have been set up for a clear explanation of the nature of the Papacy and its basis in Scripture.
The context of the Old Testament Reading should be explained. During the lifetime of the prophet Isaiah, the royal steward of the palace, a certain Shebna, was arrogating to himself royal privileges. In particular, he was having a tomb cut for himself in the area reserved for the royal Sons of David. Like Denethor in The Return of the King , he was forgetting his place as steward and confusing his role with that of the king (not an accidental parallel, by the way--Tolkien was Catholic). As a result, the LORD sends an oracle to Shebna via Isaiah, to the effect that Shebna will be replaced in his position by a more righteous man, a certain Eliakim son of Hilkiah:
Thus says the LORD to Shebna, master of the palace:
"I will thrust you from your office
and pull you down from your station.
On that day I will summon my servant
Eliakim, son of Hilkiah;
I will clothe him with your robe,
and gird him with your sash,
and give over to him your authority.
He shall be a father to the inhabitants of Jerusalem,
and to the house of Judah.
I will place the key of the House of David on Eliakim's shoulder;
when he opens, no one shall shut
when he shuts, no one shall open.
I will fix him like a peg in a sure spot,
to be a place of honor for his family."
The role of master of the palace, literally the one over the house (Heb. asher al-habayith), was the Number Two position of authority after the King (observe the dynamic in 1 Kings 18:1-5, for example). The office was first established by Solomon (1 Kings 4:6). Apparently, the badge of his office was the wearing of the key to the palace on the shoulder (Isa 22:22). The steward controlled access to the king, either by unlocking or locking the palace doors to those who sought the kings presence.
Michael Barber has done work showing that the Royal Steward was understood as a priestly character. I cannot repeat all his evidence, but I will point out the connections of which I am aware: (1) the girdle (Heb. abnet) mentioned in the passage (sash in the Lectionary translation) is only mentioned elsewhere in the OT as a priestly garment, usually along with the robe (Heb. kuttonet): Ex. 28:4, 39, 40; 29:9; 39:29; Lev. 8:7, 13; 16:4. (2) The steward is said to be a father to the House of Judah. Father is a priestly title in the Old Testament (Gen 45:8; Judg 17:10; 18:19). (3) Eliakim is the son of Hilkiah. Although we are not sure which Hilkiah this is, it is notable that the name Hilkiah is only used by Levites in the Old Testament (Jeremiah, a Levite, is also son of Hilkiah, Jer 1:1), and at least two Hilkiahs were in fact High Priests (2 Kings 22:4 etc. and parallels in 2 Chron 34; Neh 12:7).
In summary, the Kingdom of David included the office of the Royal Steward (asher al-habayit), a position associated with priesthood and second only to the king in authority.
As we move toward todays Gospel reading, lets not forget that the Gospels of Matthew and Luke both take great pains in their opening chapters to emphasize Jesus royal Davidic lineage. He is the Son of David come to fulfill all the promises of the Davidic Covenant (see Jer 33:15, 19-21). However, we as Christian readers usually practice a sort of literary schizophrenia when reading the Gospels. We do not connect the "Kingdom of David" promised to Jesus with the "Kingdom of Heaven" that Jesus proclaims in his ministry:
Luke 1:31 And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus. 32 He will be great, and will be called the Son of the Most High; and the Lord God will give to him the throne of his father David, 33 and he will reign over the house of Jacob for ever; and of his kingdom there will be no end.
Matt 4:17 From that time Jesus began to preach, saying, Repent, for the kingdom of heaven is at hand.
However, already in the Old Testament, there was an awareness that the Kingdom of David was a manifestation of Gods own Kingdom:
2Chr. 13:8 And now you think to withstand the kingdom of the LORD in the hand of the sons of David?
The Kingdom of Heaven, manifested on earth as the Church, is also the Kingdom of David, and in its structures it reflects that Davidic heritage.
This Old Testament background elucidates the Gospel reading, a controversial one whose meaning is hotly debated because of the importance of its implications:
Jesus went into the region of Caesarea Philippi and
he asked his disciples,
"Who do people say that the Son of Man is?"
They replied, "Some say John the Baptist, others Elijah,
still others Jeremiah or one of the prophets."
He said to them, "But who do you say that I am?"
Simon Peter said in reply,
"You are the Christ, the Son of the living God."
Jesus said to him in reply,
"Blessed are you, Simon son of Jonah.
For flesh and blood has not revealed this to you, but my heavenly Father.
And so I say to you, you are Peter,
and upon this rock I will build my church,
and the gates of the netherworld shall not prevail against it.
I will give you the keys to the kingdom of heaven.
Whatever you bind on earth shall be bound in heaven;
and whatever you loose on earth shall be loosed in heaven."
Then he strictly ordered his disciples
to tell no one that he was the Christ.
Isaiah 22 is clearly the background for the promise of the keys to the Kingdom. Aside from Judges 3:23-25, which has no thematic parallels, Isaiah 22 is the only passage of the Old Testament where the word key even occurs. The thematic parallels are strong: the promise to Eliakim concerning opening and shutting is repeated to Peter, although using the terms binding and loosing. Binding and loosing were technical terms in first century Judaism referring to the authority to decide matters of halakhah (lit. the walk, i.e. the behavior or how one behaves), that is, the practical application of divine law.
Jesus did not decide all matters of the application of divine law himself. Nor did he write down a book with the answers to all controversies in this area that would ever arise in the history of the Church. He did, however, invest Peter with the authority to make decisions in this regard.
Even some non-Catholic commentators (most notably, W.F. Albright, father of American biblical archeology and Old Testament studies) recognize that, in Matt 16, Jesus is investing Peter with role of royal steward in the Kingdom that Jesus is establishing.
The Church has always held that Peters authoritylike the authority of the apostles in generalwas passed down to his successors. Otherwise, passages like Matt 16:13-20 and others which speak to us of the authority of the apostles are simply matters of historical curiosity for us: "So Jesus invested Peter and the apostles with authority over the Church. After they died, however, Jesus left no provision for the governance of the Church, so now it is every believer for him- or herself." This is the view I once held myself.
It implies that apparently Jesus didnt recognize the continuing need for authoritative leadership in the Church. Maybe Jesus thought he was going to return before the apostles died (but he was mistaken). Or maybe he thought that while the Church was small, it would need strong and visible leadership, but in subsequent generations, when it spread all over the world to a host of cultures and a host of controversies would arise, there would no longer be the need for strong and visible leadership to maintain the Churchs unity and doctrine.
Let me voice my disagreement with the above-mentioned position. I do not think Jesus made a mistake about the timing of his return, nor that he did not foresee the continuing need for leadership in the Church. The succession of subsequent generations to the authority of the apostles is already visible in Scripture itself (Acts 6:1-6; Titus 1:5; 2 Tim 2:2; 1 Peter 5:1-2).
The Church was not mistaken in understanding Peters authority to be passed to his successors. So we see, already in the first or early second century, Clement of Rome exercising a spiritual authority over churches far away from his immediate geographical jurisdiction (see 1 Clement).
The priestly and paternal roles of the Royal Steward, Peter and his successors, is reflected in titles given to the Bishop of Rome: pontifex maximus (greatest priest) and Papa or Pope, meaning Father.
He continues to authoritatively bind and loose, making decisions of halakhah for the People of God. A pertinent modern example: how does divine law apply to physical and chemical contraceptives, which were not as widely available in previous centuries? Paul VI gave an authoritative halakhic decision: they are impermissible. The decision remains universally controversial, but Christians who will not accept it, I am afraid, will find themselves voluntarily extinguishing their own communities as the generations pass.