The idea that hallucinogenics open a person up to demon's is a common christian concern, and undoubtedly one amongst other religions. However, I think this stems from a weak understanding of the nature of such substances/plants, and the sacred role of them when used properly and productively. One who doesn't understand the hallucinogen's effects, or how to guide the mental journey brought on by them, will undoubtedly condem it as "evil". Those more experienced and knowledgeable with spiritual use of entheogens will consistantly tell you that it has made them an emotionally and spiritually healthier person. Furthurmore, many (mono)theists, particularily christians, have used such substances to bring them closer to god and find peace with him/her/it.