This film is more clever than it first appears, because it operates on at least two levels. On the surface, it is a taut, well-crafted horror story about a (possible) "death cult". We have some mysterious rituals, a missing husband, and seemingly silly camp activities that may or may not have a darker purpose.
Some of this is vaguely alluded to in the opening quote from H. P. Lovecraft, and further still during the lake "reveal". The scare quotes here are just because what is revealed at this moment is entirely up to the imaginations of the viewer. A certain Lovecraft story may provide a guide, or it may be merely a coincidence or red herring. But once the big reveal comes, everything goes dark, and the suspense truly becomes horror.
The subcutaneous level is focused on a theme: the truth of religion, either this one or religion in general. Though this film really only explores the reality of one (fictional) religion, it does make us wonder: what if some religions we find strange are actually right? With so many religions in the world, it is certainly possible that one or more are correct. And if the strangest ones might be right, we ought to question our own beliefs: do we believe correctly? With so many choices, it is hard to say for sure.
Though this deeper meaning may not have been intentional, it nevertheless exists and makes the film even more interesting. The filmmakers previously had a hit with "Resolution" (2012), but all signs point to "The Endless" being an even bigger success. The film played at Tribeca Film Festival on April 21, and will no doubt be seen by a wider audience throughout 2017.