I thought it was interesting though that she was the weaker of faith imo, there are a few hints to this…
the pornography watching that she denies, saying when she dies she wants to come back as a butterfly (this is subtle, but a faithful mormon or christian would not say this), trying to leave through the disbelief door and denying her faith to escape, her comments about prayer in the climax, and if you are correct about the end, the call back to the butterflies she mentioned earlier. Meanwhile sister barnes displays some hints of being unsure in her faith, she ultimately is the one to pick the belief door and say "let us leave it up to God to decide what happens" (maybe a slight paraphrase)…
All of this though presents a very real and humanizing portrayal, which I really enjoy. I watch Sophie Thatcher in yellowjackets and I think she really steals the show, she is great. This other actress was able to very convincingly portray the shy demure mormon girl too. I believe they were both raised mormon but left the faith, making for a lot of familiarity in their performances.
Cast In The Name Of God…Ye Not Guilty

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I think there are a few interpretations that could be made…
The butterfly is references a few times, in sister paxton's chat early on, in the story told to both of them about the butterfly dream, and obviously in the last sequence. Also throughout we can see butterflies (or moths…though they really are the same) which are drawn to the lamp lights both in the living room area and in the basement. It could suggest what you said,
it could suggest she is seeing her near death experience before she is about to lose consciousness, maybe it suggests the entire thing was her butterfly dream
We do not know what happens after the credits roll, who knows?
Cast In The Name Of God…Ye Not Guilty

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