What I think this movie is about...

Rating: 8/10 (12 October 2019)

You've read all of the other reviews about the fantastic acting, stunning scenery and cinematography, and unsettling score so I will concur and skip those aspects. One thing I will add to the unsettling part, I hate gore, I read the MPAA rating reviews (minus spoilers) to know what I'm getting myself into. I thought this would be worse and almost didn't watch but I could see over half the icky parts coming and look away and I was glad I watched anyway.
I'm a big fan of smart, slow-burner suspense movies and this is the epitome of that. The spoilers talk about Dani's integration and assimilation with nature and ultimately the cult. For me the main theme and why she joins them is empathy. She goes through the most traumatic moment of her life and she's grieving w/ every part of her soul and her boyfriend awkwardly holds her in his lap, looking miserable, barely touching her. Contrasted w/ discovering his betrayal and all of the woman holding, touching, and grieving with her. This empathizing is seen in many scenes with the cult members; the cliff dive dying man, the orgy, and the final burning.
I'm sure there's many more aspects I missed and I look forward to googling other theories. But the scene where Dani's pain is finally shared and validated made me cry and, in a movie scenario at least, understand her final actions.