But even Augustine holds that Christ was referring to Peter's confession as the rock, not his person. And this is obviously the only reasonable interpretation because Christ calls Peter Satan later in the same chapter. Obviously, what he calls him refers to something he has done and not something he is. Agreed?
I disagree. The case is quite clear that Simon fundamentally became Peter in a radical, unique, and irreversible way. This transformation was because of his faith, but was unique to Peter individually.
I. Turning Simon into Peter
It is true that there is a very narrow sense in which it’s accurate to say that Simon is Peter, the Rock, because of his faith. In the same way, we might say (for example) that Obama is president because of his charisma. But when we say this, we’re giving a reason for why a certain individual was selected.
So we need to be careful with what this implies, and what it doesn't. We’re not saying that (a) that anyone with charisma automatically becomes co-president, or that (b) Obama ceases to be president anytime he’s not charming.
Likewise, Simon Peter was individually selected because of his faith. But no, that doesn’t mean that (a) anyone with faith becomes Peter, or that (b) Peter ceases to be Peter when his faith falters. This is an authentic transformative change. It’s a name he carries with him for the rest of his life (unlike “Satan,” which referred to his conduct in a specific instance, and a name Jesus never calls him again).
For example, in Galatians 2, when Paul rebukes Peter's behavior, he still calls him Cephas (the Greek transliteration of Kephas, the Aramaic word for Rock). So Peter isn’t a title that only belongs to Simon when he's on his best behavior. It's his name, just as Abram forever becomes Abraham (because of his faith), and Jacob became Israel (same reason).
Nor is it a title that anyone else ever receives. In John 11:27, St. Martha says to Jesus: “Yes, Lord, I believe that you are the Christ, the Son of God, who was to come into the world.” That’s almost verbatim what Peter confessed in Matthew 16:16. Yet Jesus doesn’t change Martha’s name to Rock, doesn’t bestow the Keys upon her, doesn’t give her the power to bind and loosen, doesn’t promise to build the Church upon her, etc. If Protestants were right, that Jesus is just building His Church upon “faith like Peter's” (which isn't what He says He's doing), then we'd expect Him to similarly bless Martha. That doesn't happen.
II. Simon:Peter :: Jesus:Christ
I'm glad that HocCogitat wants to look at the context of Matthew 16, but there's something he's missing. Here's a bit of the context, from Mt. 16:16-18:
Simon Peter answered, “You are the Christ, the Son of the living God.”What should jump out (the added emphasis helps) is that Simon identifies which Jesus is (the Christ, the Son of the Living God), and Jesus responds by identifying who Simon is (Peter, the Rock, who He'll build His Church upon).
Jesus replied, “Blessed are you, Simon son of Jonah, for this was not revealed to you by man, but by my Father in heaven. And I tell you that you are Peter, and on this rock I will build My Church, and the gates of Hades will not overcome it.”
To say that Jesus calling Simon “Rock,” “refers to something he has done and not something he is” would likewise suggest that “the Christ” refers to something Jesus will do, rather than something He Is. But Jesus is the Christ, as Peter tells us above (and others, like St. John, confirm: see John 20:31). Christ isn't just a role Jesus plays: it's a part of His Person. Likewise, Peter isn't just a role Simon plays. Simon is Peter. Jesus is Christ.
III. A Rock, and a Stumbling Stone
Get behind Me, Satan! You are a stumbling stone to Me.” As Msgr. Ronald Knox pointed out in Belief of Catholics, calling Peter a stumbling stone plays upon his new name, “Rock.”
It's a nod towards the fact that Peter is, at once, a rock upon which Jesus will build the Church, and a scandalon, a rock which gets in the way. The papacy has always possessed both of these characteristics, as can be expected of an office which is of Divine origin and oversight, but occupied by sinful man. So Peter's not conditionally Rock. He's always Rock. Sometimes that's great, sometimes it's a scandal; often, it's both at once.
IV. Misusing St. Augustine
Finally, I should note two things. First, in various writings, St. Augustine equated the Rock with Peter (individually), with Peter's faith, and with Christ. So you can proof-text Augustine for anything on this point. At a minimum, Augustine didn't find Peter's confession “obviously the only reasonable interpretation,” since he suggested two alternative interpretations.
Second and more importantly, there's no question that Augustine still believed that Christ founded the papacy with Peter as the first pope, and that this papacy continued in Rome. So to take his exegesis of Mt. 16 in this way isn’t faithful to his own views on the papacy. That's poor Patristics. I know it's a popular Protestant meme, but I think it's a misleading one.
I think that the case from Scripture is quite clear: Jesus chooses one man, Simon, to become the earthly leader of His flock. Jesus chooses him because of his faith, but this blessing stays with him both when his faith is strong and when it is weak. Others (like Martha) come to share the faith of Peter, but in doing so, they don't become the Rock upon which Jesus will build His Church.
Happy Feast of the Assumption, by the way!