What are Entheogens?
Entheogens are psychoactive substances such as Psilocybin, Mescaline, and Ahahuasca that are used as adjuncts to mystical, religious or spiritual experiences.

The use of some entheogenic substances are widely described in historical documents such as the Hindu '
Soma' of the Rig Veda, the 'Kykeon' of the Greek 'Eleusinian Mysteries' and the 'teonanactl' of the ancient Mexican Mushroom Cults.

Contemporary use of entheogens include the use of Ayahuasca by the
União do Vegetal and Santo Daime Tradition in Brazil, and Peyote by the Native American Church. Although somewhat contentious, it could be claimed that the role of Cannabis within the Rastafarian religion is that of an entheogen.

However, the concept that beneficial religious or spiritual development can result from the consumption of mind-altering substances is not readily accepted in contemporary ‘Western Society’ either on a secular or a religious level.
 
“Nothing then could be more repugnant to this cultural tradition than the notion of spiritual or psychological growth through the use of drugs.” 1
Unlike many ‘Eastern’ religions such as Buddhism, traditional Judaeo-Christian theologies lack both the concept of, and the terminology to describe, identity with or oneness with God or the universe. Epiphanies and revelations such as those reported by users of ‘entheogenic’ substances such as Psilocybin, Mescaline, Dimethyltryptamine (DMT) and Lysergic Acid Diethylamide (LSD), are viewed with suspicion and rarely given value. Aldous Huxley suggested that there was no place in the current (Western) world view for valid transcendental experience and that “to be a mystic or visionary is no longer credible”.

Despite this there is currently increasing interest in the use of hallucinogens in spiritual development. Clinical research into the use of psilocybin has recently resumed in the United States, Germany and Switzerland and on July 7th 2006 the first study of its kind in over forty years was published. Conducted in compliance with US Laws it was a result of a collaboration between the Department of Psychiatry and Behavioral Science of Johns Hopkins University in Baltimore, Maryland, and the Council for Spiritual Practice based in California. The results of their research can be read in their report, entitled “Psilocybin can occasion mystical-type experiences having substantial and sustained personal meaning and spiritual significance” which can be accessed here.
It provided confirmation of what Dr. Walter Panhke had concluded following his 1962 Marsh Chapel Experiment.

More information about the origins of the word entheogen is provided here.

1 WATTS, A. 1968. Psychedelics and Religious Experience in AARONSON, B. and OSMOND, H. 1970. Psychedelics. New York : Anchor Books pp. 131 - 144