The Depravity of Humans
Throughout the text, Father Gregory is forced to wrestle with his preconceived ideas concerning the depravity of man. The question is whether or not Susan is really possessed by a demon. When he finds himself doubting that this is in fact the case, Gregory must consider his own assumptions about a person's capacity for evil. Although he does not outright tell Susan or her father that he believes she's a responsible, he does privately conclude that she is. He adjusts his beliefs accordingly, meaning he starts believing that all people hold some measure of internal evil for which they are responsible. His views are completely compatible with his religion. In fact, Bishop Crimmings encourages him to arrive at that very conclusion.
Gossip
Susan's case is a fascinating, but that ends up being a detriment to her healthy recovery. Since people are so interested in what's happening to her, they spread all kinds of crazy rumors which influence Susan herself, her father, and Father Gregory. In such a delicate predicament clear-mindedness is their most valuable ally, so these rumors really cost them all. The most telling example is Mr. Garth's friend from the village. He's an anti-Catholic occultist who gathers stories of atrocities within the church for his personal enjoyment and to add to his repertoire of anti-Catholic pamphlets. Due to his influence, Mr. Garth becomes persuaded that Susan is possessed and that somehow the church is responsible for it.
Domestic Violence & Abuse
Susan is a troubled teen to begin with, more so than normal. As the case progresses, its revealed that Mr. Garth beats her regularly. His particular brand of authoritarian parenting advocates corporal punishment for even small infractions, which results in a tense living environment for the girl. She doesn't trust her father nor feel like he has her best interests in mind. Naturally this abuse manifests adversely in her psychology which in turn opens her up to negative spiritual energies which doubtless at least partially result in her bizarre condition. As a priest, Father Gregory has been trained to detect and respond to abuse, so he immediately connects Susan's domestic abuse to her over-active provocative sexuality.
Horror
From start to finish, The Case Against Satan is a horror novel. Russel writes in a manner which builds tension chapters ahead of a dramatic incident. By never really clearing up the suspicion of possession, even at the end, he allows readers to draw their own frightening conclusions. Not only is the subject matter disturbing but Russel includes dramatic imagery which is sure to give anyone chills. Susan becomes a living vessel for the seemingly endless torture of the people around her, and the most important part is that all of her actions are completely plausible. That's the truly disturbing affect of horror as a genre; any of it is possible.
