The Unknown God is the first documentary study of Thelema, a twentieth-century religious movement in the "magical" family, founded by the occultist, poet, and prophet of a new age of personal freedom, Aleister Crowley (1875--1947). Martin P. Starr tells the history of the movement through the biography of its leader, Wilfred Talbot Smith (1885--1957), who took up Crowley''s plans for organizations to teach the latter''s methods in Western and Eastern esoteric traditions and his laws for a new world order, and established these systems in British Columbia and in California. Crowley provided the concepts; Smith and his associates made them take flesh, applying Crowley''s doctrine of "Do what thou wilt" and cementing it a part of the artistic and religious underground of the twentieth century. This account provides a contextual overview of the elements of the resulting bricolage of religions, which included Freemasonry, Theosophy, Rosicrucianism, Neo-Gnosticism and other related forms of esotericism, demonstrating the overlap between apparently disparate ideologies, groups, and participants. Drawing primarily on diaries and letters, Starr gives a rare and fascinating study of the contemporaneous application of Crowley''s thought, whose long trail we can see in the Satanism of Anton LaVey, the Scientology of L. Ron Hubbard, and the popularization of many forms of witchcraft, magic, and tantric practices.