"And I say to thee: That thou art Peter; and upon this rock I will build my church, and the gates of hell shall not prevail against it. And I will give to thee the keys of the kingdom of heaven. And whatsoever thou shalt bind upon earth, it shall be bound also in heaven: and whatsoever thou shalt loose upon earth, it shall be loosed also in heaven."
It's verse 19 that I want to pay particular attention to in this blog entry, however, namely because there is a significance behind the handing on of the "keys to the kingdom", and for Peter the responsibility being the key bearer entails. Let's explore the significance in layman's terms first.
When someone is given the "keys to the city" (traditionally an American custom) in contemporary times, it is because they have done something to earn them, i.e. they are awarded the keys to the city in recognition of some sort of achievement or accomplishment. It's not uncommon today for olympic athletes, humanitarians, civil servants, etc., to receive the keys to the city. In medieval times when when walled cities were guarded during the day and locked at night, key bearers could enter and leave the city as they pleased as trusted friends of city residents.
In more simpler terms, let's say I go away for a while and I need someone to look after my house while I'm gone, I'm going to give my house keys to someone I trust without reservation, right? My house holds my treasures and while I'm not physically present, I would the "best person for the job" to be my key bearer. I might even trust the person enough to have a house key cut for them so that they may enter my home any time if they so desire. A person entrusted with a key to another person's home, obviously, must be responsible and must not abuse this priveledge, much like in the case of the key bearer in medieval times.
The key bearer's role was authoritative as the honour of being the key bearer brought with it culpability.
And now, the scriptural significance of the "keys to the kingdom".
In Isaiah chapter 22, the prophet (Isaiah) laments the devastation of Judah. He foretells the deprivation of Sobna ("Shebna" in other translations), and the substitution of Eliacim ("Eliakim" in other translations) as steward of the kingdom:
"And I will drive thee out From thy station, and depose thee from thy ministry. And it shall come to pass in that day, that I will call my servant Eliacim the son of Helcias, And I will clothe him with thy robe, and will strengthen him with thy girdle, and will give thy power into his hand: and he shall be as a father to the inhabitants of Jerusalem, and to the house of Juda. And I will lay the key of the house of David upon his shoulder: and he shall open, and none shall shut: and he shall shut, and none shall open." - Isaiah 22:19-22 (D-R)
Broken down, verse by verse (http://www.scripturecatholic.com/the_church.html#the_church-III; John Salza, 2001-2007):
Isaiah 22:19 - Shebna is described as having an "office" and a "station." An office, in order for it to be an office, has successors. In order for an earthly kingdom to last, a succession of representatives is required. This was the case in the Old Covenant kingdom, and it is the case in the New Covenant kingdom which fulfills the Old Covenant. Jesus our King is in heaven, but He has appointed a chief steward over His household with a plan for a succession of representatives.
Isaiah 22:20 - In the old Davidic kingdom, Eliakim succeeds Shebna as the chief steward of the household of God. The kingdom employs a mechanism of dynastic succession. King David was dead for centuries, but his kingdom is preserved through a succession of representatives.
Isaiah 22:21 - Eliakim is called “father” or “papa” of God's people. The word Pope used by Catholics to describe the chief steward of the earthly kingdom simply means papa or father in Italian. This is why Catholics call the leader of the Church "Pope." The Pope is the father of God's people, the chief steward of the earthly kingdom and Christ's representative on earth.
Isaiah 22:22 - we see that the keys of the kingdom pass from Shebna to Eliakim. Thus, the keys are used not only as a symbol of authority, but also to facilitate succession. The keys of Christ's kingdom have passed from Peter to Linus all the way to our current Pope with an unbroken lineage for almost 2,000 years.
St. Peter, in Matthew 16:18-19 was named, under the authority of Christ the Lord himself, steward of the seat of the earthly kingdom of heaven, what would be the visible sign to the world that Christ dwells with us still and that his message is to be heard by and disciples made of all nations and that all may be baptised in the name of the Father, and of the Son, and of the Holy Spirit (Matthew 28:19).
Many anti-Catholics will argue that St. Peter is not the foundation of the church, but Christ is. This is an argument of semantics: Christ is the foundation of Christendom; Christianity. St. Peter, however, as it tells us quite clearly in Matthew 16:18, is the foundation, the rock, on which Christ builds his church!
Think of it this way: Jesus is the project manager, and St. Peter is the contractor. Jesus gives St. Peter the authority to build ("And whatsoever thou shalt bind upon earth, it shall be bound also in heaven: and whatsoever thou shalt loose upon earth, it shall be loosed also in heaven", Matthew 16:19) and St. Peter and the disciples are given instructions (Matthew 28:19-20) and guidance (John 14:23-29) to expand their developments throughout the world ("...called 'Catholic' because it extends over all the world, from one end of the earth to the other" - Cyril of Jerusalem, 315-386AD).
The key symbolises trust and authority, ergo, the keys to the kingdom of heaven are given to one that is most trusted, a "first among firsts", a most loyal and stable character, and this character is given binding authority.
There is no authority greater than that given by the Lord Jesus Christ, and for almost 2000 years the key has been passed on from one pope to the other, starting with St. Peter and now sitting with Pope Benedict XVI. This is why the Vatican's own flag has a set of keys on its canvas:
The Church on which the successor of St. Peter sits, to this day carries out the instructions given by Christ and will do so until the day of Christ's return. The Church remains a light to the world (Matthew 5:14), giving witness to the good news that Christ and the heavenly Father desires for all men (sic.) to hear and allowing his Holy Spirit to work in each of us, so that the heavenly kingdom may be filled with souls filled with love for God, made possible with the price His Son paid for us and our salvation.
"That they all may be one, as thou, Father, in me, and I in thee; that they also may be one in us; that the world may believe that thou hast sent me." - John 17:21