The Mystery Religions of the ancient world frequently, if not always, employed the use of psychoactive drugs or entheogens to induce altered states of consciousness. Such experiences were indispensible to the initiation of members, their rituals relating to spiritual development, and ultimately the attainment of the “peak” experience(s) that represented the apotheosis and fulfillment of their theological and spiritual initiations. Thus, more than any other principle, the entheogenic — i.e., pharmacological — induction of nonordinary states defines the theology and practice of the ancient Mystery Religions. Keywords: Altered states; ancient religions; drugs; Mystery Religions; Greek Mysteries; entheogens; psychedelics and sacraments