In 1903, Crowley married Rose Kelly, and they went to Egypt on their honeymoon. After returning to Cairo in early 1904, Rose began entering trance states and insisting to her husband that the God Horus was trying to contact him. As a test, Crowley took Rose to the Boulak Museum and asked her to point out Horus to him. She passed several well-known images of the god and led Aleister straight to a painted wooden funerary stele from the 26th dynasty, depicting Horus receiving a sacrifice from the deceased, a priest named Ankh-af-na-khonsu. Crowley was especially impressed by the fact that this piece was numbered 666 by the museum, a number with which he had identified since childhood.

The upshot was that he began to listen to Rose, and at her direction, on three successive days beginning April 8, 1904, he entered his chamber at noon and wrote down what he heard dictated from a shadowy presence behind him. The result was the three chapters of verse known as Liber AL vel Legis, or The Book of the Law. This book heralded the dawning of the new aeon of Horus, which would be governed by the Law of Thelema. "Thelema" is a Greek word meaning "will", and the Law of Thelema is often stated as: "Do what thou wilt". As the prophet of this new aeon, Crowley spent the rest of his life working to develop and establish Thelemic philosophy.

In 1906 Crowley rejoined George Cecil Jones in England, where they set about the task of creating a magical order to continue where the Golden Dawn had left off. They called this order the A\ A\ (Astron Argon or Silver Star), and it became the primary vehicle for the transmission of Crowley's mystical and magical training system based on the principles of Thelema.

Then in 1910 Crowley was contacted by Theodore Reuss, the head of an organization based in Germany called the Ordo Templi Orientis (O.T.O.). This group of high-ranking Freemasons claimed to have discovered the supreme secret of practical Magick, which was taught in its highest degrees. Apparently Crowley agreed, becoming a member of O.T.O. and eventually taking over as head of the order when Reuss suffered a stroke in 1921. Crowley reformulated the rites of the O.T.O. to conform them to the Law of Thelema, and vested the organization with its main purpose of establishing Thelema in the world. The order also became independent of Freemasonry (although still based on the same patterns) and opened its membership to women and men who were not masons. Today, the Mystery Schools and Secret Orders that Edward Alexander (Aleister) Crowley influenced are growing around the world, and in numbers greater than ever.