This shit is the same "no true scotman" shit that muslim use to argue that ISIS aren't muslims.
Religion dogmas and traditions play well in people which have extremely narrow worldviews and see everything as an attack on the status quo: news at eleven. Just because those people are stupid it doesn't mean that they aren't religious, i'd argue that in fact religion was created exactly in a way to appeal to those people in the first place.
A lot of Christian Americans believe in the christian god in the same way a lot of muslim in regressive nations believe in it: they use it as a way to rationalize acting like a dick toward others. Acting like they're not "true" believers is just a refusal to accept the fact that religions was created and shaped by history as a mean to control the masses in a certain way, and to preserve the status quo against the attacks over it. There's no way you can't call them true christians or whatever, if anything, i'd say history precedents tend to show religions as a conservative force against equality the vast majority of time rather than the opposite (aside from the moment they were born, where they were arguably a motor for change, altought more "neutral" than anything else, see the predicaments for helping poors, but see also the acts of fanaticisms of early christians stoning phylosophers and burning libraries, and see the improvemnt of slaves situations, but the violent conquests of the Dar-Al-Islam in Islam). "True" Christian or "True" god is such a dumb concept anyway. Trueness is applicable only to strict logic.