OP: "When it comes to something like communism, popular opinion seems content to say that communism in practice is not like it is in theory, yet when the tables are turned on popular opinion's favourite religion, Christianity, you hear more talk about "oh, that is because [x] are not true Christians" or whatever."
I'm sorry, but you seem to be overgeneralizing. What evidence to you have that arguments along the lines of "They aren't true Christians" are more common than "They are just different"?
Saying that one is Christian is a statement of intent, not an evaluation of one's actual behavior. It's like saying you are a good person- one bad behavior, or even a series of them, doesn't invalidate the statement of good intent.
As for differences between different sects- a goodly number of ordinary Christians are willing to concede that members of other religions entirely will be allowed into heaven, and aren't doing anything wrong, may even be following their own calling- what are we going to say about members of other Christian denominations?
I don't think anyone has the right to evaluate who is or isn't a Christian. After death, there will be a much better-informed judge of that than anyone living on earth, presuming it even matters, so my advice to my fellow Christians would be to let it be.
(Cue Beatles music track)
“Disobedience is the true foundation of liberty. The obedient must be slaves.”