The Greek verb katischuo (from kata- + ischuo, to be strong) means to be strong enough to overpower, to prevail against, or to overcome. It appears in Matthew 16:18 — Jesus' declaration that the gates of Hades will not katischusousin (prevail against) His Church — and in Luke 21:36 (to have strength to escape coming judgment) and Luke 23:23 (the crowd's voices 'prevailed').
Matthew 16:18 is one of the most encouraging declarations in Scripture: the gates of Hades will not katischuo against the Church. The imagery is significant: gates are defensive, not offensive — the Church is the advancing army, pressing against the stronghold of death. The gates of Hades cannot hold their captives when the Church, built on the confession that Jesus is the Christ, advances. This is the same power Paul describes in 2 Corinthians 10:4: 'The weapons we fight with... have divine power to demolish strongholds.'