The exorcist must be some of the most devoted Christian on the planet. If someone is believing something that much, they can't be considered a fair or reliable observer. Even a staunch atheist when pressured enough and presented with an ambiguous situation (e.g. a weird picture where the shape and shadows just conveniently form a paranormal figure) would start to make false assumptions. Now make it a priest whose entire life is spent on seeing "demons" and I would not be surprised if these guys are going to hallucinate while waking.