In his 1972 book, Satan is Alive and Well on Planet Earth, Hal Lindsey, offers a number of tips for "diagnosing demonic possession." Here is a rundown of some of his telltale signs:
- Occult Powers- "Be wary of those who have psychic powers," he writes. The ability to foretell the future could be a "gift of discernment from God," but can also be a "counterfeit from Satan."
- Necromancy- Mediumistic communications with the dead are generally a "demonic deception," whereby the demon is impersonating the deceased person.
- Change in Intelligence- When a demon is exerting influence over an individual there may be a "sudden or sporadic increase in the person's intelligence." He cites Hitler as an example.
- Increased Strength- "I saw a demon-possessed man at Berkeley with such superhuman power it took six men to hold him down," Lindsey notes.
- "Filthy" Uncontrolled Swearing- The compulsion to "blaspheme the name of God and Christ" can in tandem with other symptoms be a sign of demonic influence.