There are the following tags related to Christianity (in alphabetical order):
The first one and the last two are obvious (there is also broad religion tag). I was thinking about the church. At the moment it's excerpt says
Questions related to the Catholic church in history
but this is in fact what the catholic-church is intended for. If we consider as Church all Christian movement since the death of Jesus, the christianity is enough. I propose that we make an alias in "church" to "catholic-church" or to "christianity".
I looked at questions tagged with church and think that christianity would be the best one to be redirected to.
Or maybe "church" is intended to be about Christian places of worship, but no question is about this kind of building.