A couple of disjointed comments @F250 on that verse: "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."
I have heard it said that the "rock" that Jesus says he will build his church on, is the rock of confession-- that we confess Jesus is the Messiah (which is what Peter had just done and 16:18 was in response to that confession/posture). Matthew 16:16-17 Peter replied, “You are the Christ, the Son of the living God.”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.
That said, I have also heard it said that Jesus was one for nicknames. He nicknamed James and John "Sons of Thunder" and more famously Simon was renamed as Peter (Greek Petros means Rock) in what I have heard would be a modern day equivilent of giving your buddy the nickname "Rocky" or someone bull-headed, strong, stubbon but also heart of gold, underdog and loveable....like the eponymously named boxing movie.
Lastly I've heard that verse used as an invitation for Christians to take up their cross and not be fearful, but be on the attack-- be on the offensive. For Gates are a defensive barriers. A lot of folks get it backwards in their minds that the Christianity and faith (and evangelism) is something to hide behind like gates, as the forces of evil push against it. But it's quite the opposite. We are called to push against the wicked and sinful and storm those gates with the righteousness and steadfastness of faith, etc. etc.
Or so that's how I've heard it said.