About

"This is what X actually believe" is a meme based on an episode of South Park, usually as part of an image macro, showing a perceived nonsensical aspect of whatever X is. To this end, X is typically related to religion (as from its original use) or politics, but can extend to other areas.

Origin

"Trapped in the Closer" is a critically-acclaimed episode from the 9th season of South Park. The episode revolves around Scientology, centering on Stan being considered the reincarnation of L. Ron Hubbard by the group, and thus exalted as their leader. As such, several scenes of the episode are used to explain the teachings of Scientology – which itself involves some rather strange concepts. As both Matt Stone and Trey Parker are vocal opponents of Scientology, this was given with as much seriousness as they could give it, with the text "THIS IS WHAT SCIENTOLOGISTS ACTUALLY BELIEVE" shown over the images.

The episode, unsurprisingly, was met with controversy from Scientology, leading to "Closetgate" that only served to heighten interest in the episode, and Isaac Hayes' departure from the show. The episode was nominated for an Emmy that year, but lost to The Simpsons.

Meme

Since it's airing, Internet users have parodied the presentation of the background of Scientology for other religions, political affiliations, and the like. The phrase is usually attached to an image, text, or video of part of the targeted group's beliefs, which, when taken out of context, would be difficult to understand or believe without further explanation. This, in some form, is similar to the I'm just going to leave this here meme to let the reader to decide the ridiculousness of the stated belief.

Common examples are use of existing videos from religions, simply titling them "This is what X actually believe", mimicking the style in the South Park episode:

"This is what Jehovah's Witnesses actually believe"

"This is what Mormons actually believe"

And of course, there's parodies and remixes of other materials…
"This is what conspiracy theorists actually believe"

*"What creationists actually believe"