In the 1840s in upstate New York arose a belief that it is possible to communicate with the spirits of the dead. Sessions (séances) were held by "mediums" (intermediaries) who would convey information back and forth. The primary influences and methods of this system were derived from Mesmer and hypnotism, a technique that manipulates the sleeping consciousness, which is black magic. Mediumism is a technique of black magic that is more commonly known now as channelling. Channellers are either cunning hoaxes or outright states of possession by demons. No genuine spiritual master needs to communicate through the body of another person.
"We have frequently heard of so many unbalanced individuals who state that they “channel” entities from beyond! Usually, those “channelers” are mediums." - Samael Aun Weor, The Divine Science
"Any Master of Samadhi can clearly verify when in the state of ecstasy the following: that which is manifested through the mediums of Spiritism are not the souls or the spirits of the dead, but the devil-“I’s” of the dead, which are the psychic aggregates that continue beyond the burial chamber. It has been said to us with much emphasis that during the postmortem (after death) states the mediums continue to convert themselves into beings who are possessed by a demon or demons. It is unquestionable that after a certain time the mediums conclude by divorcing themselves from their own divine Being. Then they enter into the submerged devolution of the infernal worlds." - Samael Aun Weor, The Three Mountains
"Remember that spiritualist mediums often serve as vehicles for black entities. These entities pose as saints and advise against the perfect marriage. Usually, they declare themselves Jesus Christ or Buddha, etc., to cheat the fools." - Samael Aun Weor, The Perfect Matrimony
"Numerous people with mediumistic faculties exist within multitudes of spiritualist temples, where multitudes of impostors manifest through the mediumistic faculties of these “chanellers.” All of them claim to be “channeling” Jesus Christ and the wretched people firmly believe in such imposters. The abyss is full of tenebrous entities, which manifest themselves through the faculties of the medium-chanellers within these spiritualist temples, where their wretched devotees are sincere but unfortunately mistaken people filled with terrible fanaticism. The leaders of those spiritualist centers exploit the Christ with good intentions." - Samael Aun Weor, The Major Mysteries
"Spiritualists will try to confuse your mind in order to convince you that their mediumistic sessions are the best of all, ignoring that in all their spiritualist centers, repugnant and horrible demons of the abyss usually present themselves as ineffable saints or as Jesus Christ in person. Regrettably, these wretched medium-chanellers are victims of repugnant larvae and demons of the abyss, and the gravest of all is that they are convinced that they are in the light; no tenebrous individual believes that he is doing iniquitous acts." - Samael Aun Weor, The Major Mysteries