Hi FD,
When we look to see if the 'gates of hell' have prevailed against the 'church', we're going to need to define exactly what 'church' refers to. Is the 'church' some worldly defined group of people who gather together for a particular worship service, or is 'church' the body of people who are born again? If we define 'church' by the second definition, then we are assured that the gates of hell have not prevailed against them. It has always been my understanding that 'church' is translated from ekkelesia and that means those called out from a larger group. Therefore, I don't consider the catholic organization or the baptist organization or the methodist organization or the presbyterian organization to be the 'church' that Jesus and the new covenant authors were writing to and about. For me, the church is all those who have been born of the Spirit of God. There will be some of those in many of the denominations and the gates of hell have not prevailed against them. They have certainly tried, but they have not prevailed.
If we use the first definition, then yes, the visible, worldly body of people who gather under the name of some denomination has certainly been splintered, but they do still stand and so neither have the gates of hell prevailed against them.
God bless you.
In Christ, Ted