You are incorrect on "the gates of hell would not stand against his church", in the sense you interpret that.
Jesus asked Peter "Who am I?" and Peter replied "You are the Messiah, the son of the God"...(short version)
And Jesus said ""Flesh and blood hath not revealed it to you but the Father, etc and to Peter he says "Upon this rock I will build my church and the gates of hell will not prevail against it". THAT ROCK was NOT Peter BUT the REVELATION given to Peter from God of Who JESUS was!
The "falling away" is not the rapture but the actual "falling away" of many of God's people, there is no once saved, always saved salvation. IT's in the Book of Hebrews....look it up. Somewhere between Chapters 6-12 I believe. Many had already fallen away, in fact on 9/11 many people went to church in droves, and have dropped away again.