Great acting work all round, Elizabeth Olsen in particular, but I wasn't that smitten with the execution. The cult subject matter seemed to give writer/director Sean Durkin carte blanche to ignore logic or rest lazily on horror movie tropes rather than putting effort into creating something of substance.
So the big reason for Olsen's exit from the cult is a cheesy horror film cliché rather than something less overt and therefore more insidiously effective. And her character's erratic behaviour, apparently through cult indoctrination, is only fitfully believable (would she really, as someone who has spent at most two years in this environment, have forgotten everything about her previous experiences of society, such that she thinks its normal to lie at the bottom of her sister and husband's bed while they are making carnal?). She is oblivious or reacts as the audience would according to the scene's requirements, and Durkin is only able to sketch in the broadest of strokes.
The sound design also has a big sign around its neck saying "THIS IS DISTURBING". There's a point where the flashback structure becomes repetitive rather than intriguingly oblique. And all this leads to an ambiguous conclusion that may be in keeping with the themes of the film (she cannot escape what it has done to her, not matter what happens) but feels tiresomely de rigueur for this type of fare. I'm being a bit too down on it, maybe; Olsen was luminous, and Sarah Poulson very good as her sister. John Hawkes can do this kind of creep in his sleep, though.