Demon
The contemporary Roman Catholic Church unequivocally teaches that angels and demons are real beings rather than just symbolic devices. The Catholic Church has a cadre of officially sanctioned exorcists which perform many exorcisms each year. The exorcists of the Catholic Church teach that demons attack humans continually but that afflicted persons can be effectively healed and protected either by the formal rite of exorcism, authorized to be performed only by bishops and those they designate, or by prayers of deliverance, which any Christian can offer for themselves or others.
The Exorcist (novel)
After several unsuccessful psychiatric and medical treatments, Regan's mother, an atheist, turns to a local Jesuit priest for help as Regan's personality becomes increasingly disturbed. Father Damien Karras, who is currently going through a crisis of faith coupled with the loss of his mother, agrees to see Regan as a psychiatrist, but initially resists the notion that it is an actual demonic possession. After a few meetings with the child, now completely inhabited by a diabolical personality, he turns to the local bishop for permission to perform an exorcism on the child.