Hellmouth

13 Now when Jesus came into the district of Caesarea Philippi, he asked his disciples, “Who do people say that the Son of Man is?” 14 And they said, “Some say John the Baptist, others say Elijah, and others Jeremiah or one of the prophets.” 15 He said to them, “But who do you say that I am?” 16 Simon Peter replied, “You are the Christ, the Son of the living God.” 17 And Jesus answered him, “Blessed are you, Simon Bar-Jonah! For flesh and blood has not revealed this to you, but my Father who is in heaven. 18 And I tell you, you are Peter, and on this rock I will build my church, and the gates of hell shall not prevail against it. 19 I will give you the keys of the kingdom of heaven, and whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loose in heaven” (Mt 16:13-19).

I find commentaries on this passage to be unsatisfactory overall, so I'm going to take a stab at my own interpretation. 

1. Let's begin with some programmatic questions:

i) What does the "rock" refer to?

ii) Does Hades refer to the realm of the dead or the realm of the demonic?

iii) Does binding/loosing have independent meaning, or is that simply an alternative metaphor for keys, and derives its meaning from whatever the keys represent?

iv) Are the gates of hell and the keys of heaven mutually interpretive, or does the latter have an independent meaning?

2. Now I propose answer my own questions:

i) Caesarea Philippi is situated on a rocky terrace at the base of Mt. Hermon. As such, it's natural to suppose the rocky metaphor was suggested by the immediate surroundings. Jesus was standing on rocky ground, and standing in the shadow of Mt. Hermon, at the time he made his statement. 

This may also goes to a difference between the written word and the spoken word. Consider the demonstrative pronoun: "this". In that setting, it's easy to imagine him pointing to an actual rocky object. "I will build my church on this!"–accompanied by an illustrative gesture. The repetition of "rock" may well include a reference to Simon, but the double reference may also include a reference to the rocky surroundings. Indeed, that may be primary. 

"Rock" is probably a double entendre, both for Peter and especially the emblematic location. "Rocky" is a pun in honor of Peter's insightful confession, but what the church is built on is what the location symbolizes. 

ii) In Revelation, the Netherworld is subdivided into a realm of the dead (Rev 20:13-14) and a realm of the demonic (9:1-11; 11:17; 17:8). And keys are associated with each (Rev 1:18; 9:1-2.; cf. 20:1-3). My point is not to use Revelation to interpret Mt 16 directly. Rather, this seems to be stock imagery that was in circulation in Jewish circles. 

iii) Caesarea Philippi was pagan territory. In OT times, it may well have been a site of Baal-worship. Later on, it was a shrine for the Greek god Pan. So it would have demonic associations.

iv) Although Matthew doesn't fill in the details, the implicit imagery involves a parallel between hades and heaven, where they stand in contrast. Gates imply keys and keys imply gates. If we mentally flesh it out, the reader should visualize both heaven and hades as gated locations.

Since these two images occur back-to-back, not to mention the intrinsically related imagery, it stands to reason that these are mutually interpretive, picturesque metaphors. And it would be jarring if binding/loosing had a different import. 

v) Gates can be used to lock people out or keep people from escaping. A form of authorized access and/or confinement. The porter is a sentinel who guards the site. No one can enter or leave unless he unlocks the gate. 

vi) Given the associations with heathen idolatry, I think hades more likely connotes the realm of the demonic in this evocative setting. Jesus may be boldly saying he will build his church on top of hellmouth. The gates of heaven and hades may not be two separate gates, but a single gate separating the church from the demonic realm. And the function of the gate may be to block the demonic realm from storming the church. It's daring to build the church right over hell, but that's an example of God subjugating his enemies. A variation on making his enemies his footstool. Rather than building his church at a safe distance, he builds his church right behind enemy lines to demonstrate God's invincibility. The church survive and thrives in the face of the enemy.