Three Readings of Matthew 16:18’s “πυλαι αδου”
“on this rock I will build my church, and the gates of [hades / hell / sheol] will not [prevail against / overpower, overcome, overrun / dominate] it [the church].”
- The Defensive Church
As the phrase “gates of Hades” is used in Matt 16:18, it seems to include not just the city of the dead itself but also its inhabitants, especially its demonic rulers. […]The image in Matthew is of the rulers of the underworld bursting forward from the gates of their heavily guarded, walled city to attack God’s people on earth.
When we speak of demonic powers flooding the earth, we are speaking the language of Jewish apocalyptic. Jewish apocalyptists believed that, in the end-time, the powers of cosmic chaos, restrained since creation, would break forth from their restraint and bring unparalleled tribulation upon the world. 
[…] In the age inaugurated by Jesus’ death and resurrection, the gates of the underworld will swing open and the horrors of the pit will erupt onto the earth with a roar, attacking everything on it—including the church—with unbridled fury. In the midst of this peril, however, Peter will be given the keys that unlock the gates of heaven. Those gates, too, will swing open, and the kingly power of God will break forth from heaven to enter the arena against the demons. Hades will not prevail against the church because God will be powerfully at work in it, revealing his purposes for it and imparting the heavenly power to fulfill those purposes, so that his will is done on earth as it is in heaven. 
from Joel Marcus, “The Gates of Hades and the Keys of the Kingdom,” CBQ (1988).
2. The Offensive Church
For most people, I suspect, the church is pictured in their minds as a strong enclosure, a mighty fortress, that devotes most of its time and energy to defending itself against the assaults of wickedness and evil. The church, surrounded by thick walls, just sits there on a high mountain, while the powers of hell creep up form the murky valleys around it, trying to invade and destroy it. Most people imagine the church as the defender and evil as the aggressor. Evil is loose in the world and the church is scared to death.
That is backwards, according to this text. “The gates of Hades will not prevail against it,” Jesus says. The gates of hell will not prevail against the church. Think what that means. The church is the aggressor, according to Jesus, and hell is scared to death. Hell throws up enormous walls with strong gates to protect itself against the grace of God, but no walls or gates that hell can erect will ever keep the church out. The church’s assignment is to go to hell and attack it, and Jesus’ promise is that hell will eventually be defeated by the church and its Gospel.
from Ed Peterman, Practically Preaching: When Did We See You? Sermons for Year A Matthew, (2004), 210.
3. The Immortal Church
“The gates of Hades” is a metaphor for death. …The “gates” thus represent the imprisoning power of death: death will not be able to imprison and hold the church of the living God. …The imagery is rather of death being unable to swallow up the new community which Jesus is building. It will never be destroyed.”
from R. T. France, The Gospel of Matthew (Eerdmans, 2007), 624-5.