What is the theme in The Demonologist: The Extraordinary Career of Ed and Lorraine Warren by Gerald Brittle?

Demonology plays a vital role as it's the prime focus of the book. Demonology, as explained by those who study it, is concerned only with supernatural events. They seek unnatural behavior, and seek to find solutions for the people involved in the spiritual event. Demonologists seek to send earthly spirits to their resting place, and to send demonic spirits, with the help of clergymen, back to their own realm. The demonologist arrives expecting to find unnatural events explainable by natural circumstances.