The claim that “this person/group does not understand their own religion” should be eliminated from academic prose. If we think someone misunderstands their religion, it’s we who misunderstand.
Of course it’s clear that many Christians don’t know the history of Christianity, and many Muslims don’t know the history of Islam, just as many Americans don’t know much about the history of the US.
However, imagine someone saying that the 2nd amendment protects the right to bear arms, including semi-automatic assault rifles. While that’s clearly not what the 2nd amendment meant for the framers of the constitution, it would be stupid of us to say that this individual doesn’t understand their own politics. They understand their politics quite well.
When we say that someone misunderstands their own tradition, what we’re doing is constructing an authentic history and placing this person outside of it.
I often hear my students accuse their nominally Catholic peers of not understanding Catholicism. By contrast, I’d say most Catholics are nominally Catholic, and that they therefore represent the majority or the center (at least on the northeast United States). And my nominally Catholic students understand nominal Catholicism expertly.