Interpreting a film for symbols and themes is a somewhat subjective process. I liken the interpretive process to an asymptotic curve.
In Mike Flanagan's Absentia, the Christianity is subtle but explicit. Buddhism also makes an appearance. Why are these religions in the film? Do they serves a function?
Absentia is about two sisters, Trish and Kelly, who live near a portal to another dimension. A dimension inhabited by Lovecraftian demons. The sisters are unaware of this. Or that it was a demon that abducted Trish's husband seven years ago.
Trish is a Buddhist. Kelly is a Christian. Trish meditates with incense and a gong. Kelly prays before a crucifix on her bedroom wall. The sisters occasionally discuss their respective religions. Thus, Absentia's religious elements are explicit.
But not much is made of these elements. The sisters' discussions are brief and without conflict. Thus, while the religious elements are explicit, they are also subtle.
How do these religious elements serve the film?
Eventually, the demon abducts Trish. Kelly offers herself up to the demon in exchange for Trish. At the last minute, Kelly changes her mind and flees. To no avail. The demon takes Kelly -- and keeps Trish.
Actress Katie Parker (who plays Kelly) suggested at a recent screening that Kelly's Christianity and Trish's Buddhism were intended to emphasize the monster's power -- that no religion could protect you.
Yes, I can see that interpretation. But I also see how Absentia's religious elements function in other ways:
1. Kelly's explicit Christianity adds subtext to the story. It fleshes out her character.
2. Her Christianity provides motivation for her character when she offers to sacrifice herself for Trish. That Kelly changes her mind and flees demonstrates that she's still human and flawed (i.e., only Christ is perfect.).
Apart from her Christianity, two other factors motivate Kelly's intended self-sacrifice: 1. Her love for Trish, and 2. Her hurt and guilt over Trish having called her a prodigal sister. Kelly is hurt by this comment and wants to prove Trish wrong. And Kelly feels latent guilt; a part of her worries that there's truth to Trish's accusation.
As for Trish's Buddhism, I think it's there to offset Kelly's Christianity, for two reasons.
1. Some viewers may think the film too preachy if Christianity is the only religion portrayed in an explicitly positive manner.
2. Some viewers regard Christians as bigots. This preconception is proven wrong by Kelly accepting her Buddhist sister. Thus, Kelly's reaction to Trish's religion further fleshes out Kelly's character (i.e., she is a humble and tolerant Christian).
Trish's Buddhism acts as foil for Kelly's Christianity (i.e., Trish's Buddhism functions more to flesh out Kelly than Trish).
For more about interpreting themes and symbols in horror films, see Horror Film Aesthetics: Creating the Visual Language of Fear. This blog represents a continuing discussion of my views on horror, picking up from where the book left off.