Well, given the centuries and number of people and things around the globe that fit some kind of Christianity, yes I expect the meaning of the word to be pretty

Well, given the centuries and number of people and things around the globe that fit some kind of Christianity, yes I expect the meaning of the word to be pretty diluted. That's the same logic regarding any concept at that scale ("American" would be the same case, there is a tight definition, but the wider usage has almost nothing to do with it) To me that's just how language works.