Aleister Crowley was an English occultist, ceremonial magician, poet, painter, novelist, and mountaineer. He founded the religion of Thelema, identifying himself as the prophet entrusted with guiding humanity into the Æon of Horus in the early 20th century.

Born to a wealthy Plymouth Brethren family in Royal Leamington Spa, Warwickshire, Crowley rejected this fundamentalist Christian faith to pursue an interest in Western esotericism. He was educated at the University of Cambridge, where he focused his attentions on mountaineering and poetry, resulting in several publications. In 1898 he joined the esoteric Hermetic Order of the Golden Dawn, where he was trained in ceremonial magic by Samuel Liddell MacGregor Mathers and Allan Bennett. Moving to Boleskine House by Loch Ness in Scotland, he went mountaineering in Mexico with Oscar Eckenstein, before studying Hindu and Buddhist practices in India. He married Rose Edith Kelly and in 1904 they honeymooned in Cairo, Egypt, where Crowley claimed to have been contacted by a supernatural entity named Aiwass, who provided him with The Book of the Law, a sacred text that served as the basis for Thelema. Announcing the start of the Æon of Horus, The Book declared that its followers should adhere to the code of "Do what thou wilt" and seek to align themselves with their Will through the practice of magick.
After an unsuccessful attempt to climb Kanchenjunga and a visit to India and China, Crowley returned to Britain, where he attracted attention as a prolific author of poetry, novels, and occult literature. In 1907, he and George Cecil Jones co-founded a Thelemite order, the A¿A¿, through which they propagated the religion. After spending time in Algeria, in 1912 he was initiated into another esoteric order, the German-based Ordo Templi Orientis (O.T.O.), rising to become the leader of its British branch, which he reformulated in accordance with his Thelemite beliefs. In 1920 he established the Abbey of Thelema, a religious commune in Cefalù, Sicily where he lived with various followers. His libertine lifestyle led to denunciations in the British press, and the Italian government evicted him in 1923. He divided the following two decades between France, Germany, and England, and continued to promote Thelema until his death.