I have unfortunately not seen it, though have heard about it, from what I've heard somewhere it seems this is more focused upon the trial of a priest whom killed a girl in the name of exorcising a demon out of her body? Either that or that was the true story it was based on.

Now what I don't get is, why would a demon be so bored enough as to go all the way out as to take control over our body, hell, the majority of humans sin anyways, consciously or unconsciously and mostly don't give a shit or make up excuses. Plus we're just lowly humans whilst these are creatures strong enough to go and defy god and nearly succeed whilst us we would be wiped out of existance before we'd breach the first gate.

that's my two cents, sorry it's a bit off.