meh, psychosis is just a word for a thinking abnormality. in some cases there are clinical explanations for it; in others there aren't.
don't you find it just a tad bit peculiar that she couldn't walk past a Jesus statue? that she refused to drink holy water?
or even this...
he priests produced recordings of various exorcisms they had performed, during which they said they could distinguish the voices of two demons in the process of arguing, wondering which of them would leave the body of Anneliese first.