All these religious posts are wrong. And right.
Emily goes about her existence frying eggs, waking up, using the bathroom, going to sleep all to do it all over again. With one pain, and one question. "Everyone left me. WHY?"
Emily is the symbol for mankind, self-aware creatures of routine with unanswered questions that threaten their very existence. Sylvia is the charlatan (think Ricky Gervais in THE INVENTION OF LYING) who, when hired by an authoritative group (the living family), must quell the unease of the spirit by telling them to walk into the light.
As the story progresses, we have glimpses, flashes and hints to what the movie is leading up to. And if you're quick, you'll solve the whole movie in the first 30 minutes. (I was not.) Emily asks Sylvia, "A light? It's as it's been written?" This is a reference to mentions of transcendence in the Bible and her doubt in the existence of a heaven or hell. After Sylvia calms her down halfway through the film, Emily tells Sylvia that she has a gift, the first sign of caving into the idea of a God. Later, when the demon tells her that her God is dead, it is followed by a scene where Emily opens a door to nothing. Not blackness, not darkness, but nothing. The idea of NOTHING after death scares Emily so badly that she retreats back into the purgatory she's been living in.
When the house begins to fall apart and get engulfed in the ooze (POLTERGEIST, anyone?) Emily has no choice but to rely on Sylvia for help, but Sylvia has turned her back on her. Like a clergyperson who is in actuality a political pawn, Sylvia has succeeded in getting rid of Emily by any means necessary and has done her job according to the family. Emily has been split into two, now, the male part and the female part and they both sob, knowing that "the light" has been a lie. "Sylvia, there is no light! Everything is getting dark!"
In the end, the psychic was hired by the family to get rid of the souls that lived in that house, and she did so with deceit. I think the film, especially the dream sequences and the ending, are a statement on the deceitful nature of organized religion in general.