That should really be "gates of Hades" since it applies to Hades, not to Hell. So what did the Lord mean by this?
And I say also unto thee, That thou art Peter [Gk Petros = stone or boulder], and upon this rock [Gk Petra = cliff or large mass of rock] I will build my church; and the gates of hell [Hades] shall not prevail against it.
1. Christ clearly distinguished Himself -- the Rock as He is called in Scripture -- from Peter or Petros, a stone or boulder to ensure that there would be no misunderstanding about whom the Church is built upon. Other foundation can no man lay than that is laid, which is Christ.
2. The Church -- redeemed and regenerated children of God -- is indwelt by Christ and the Holy Spirit, and that is why the Church is the Body, the Bride, and the Building of Christ. It is definitely not the Catholic Church or any other denominational church.
3. How do the gates of Hades not prevail over (Gk katischusousin = overpower) or overpower the Church? We need to go back to the fact that the OT saints were held *prisoners* in Sheol/Hades until the resurrection of Christ. But Christ went to Hades for three days and three nights, and then took all the OT saints to Heaven after His resurrection. They are now in the New Jerusalem (Heb 12:22-24).
4. It was at that time that the metaphorical gates of Hades were overthrown or knocked down by Christ, and ever since no saint has entered into Hades, but all have gone to be with Christ since He ascended to Heaven. Thus the gates of Hades are powerless over the children of God, and only the unsaved souls and spirits are now in Hades.