part of the problem here is that "christianity" is impossibly vague. the different denominations are drastically different. you can't just throw "christianity" out there like it has some encompassing meaning. you've gotta be a lot more specific.
it's really just a buzz word. and people will assign just about whatever they want to it.
that's not recent, either. it's not just post-reformation. they had heretics all over the place from day one. it took them several centuries just to come up with a basic creed, and that didn't even really stick. nobody's really ever been sure what the term means.