The Gnostic Mass is the central ritual, public and private, of the O.T.O
Aleister Crowley wrote Liber XV, the Gnostic Mass, in 1913 while travelling in Moscow, Russia. The structure corresponds to the Mass of the Roman Catholic Church, replacing Christian tenets of faith with the principles of Thelema. It is the central rite of Ordo Templi Orientis and its ecclesiastical arm, Ecclesia Gnostica Catholica.
The ceremony calls for five officers: a Priest, a Priestess, a Deacon, and two acolytes, called 'Children.' The end of the ritual culminates in the consummation of the eucharist, consisting of a goblet of wine and a Cake of Light, after which the congregant proclaims “There is no part of me that is not of the gods!”
Crowley explains why he wrote the Gnostic Mass in his Confessions:
"While dealing with this subject I may as well outline its scope completely. Human nature demands (in the case of most people) the satisfaction of the religious instinct, and, to very many, this may best be done by ceremonial means. I wished therefore to construct a ritual through which people might enter into ecstasy as they have always done under the influence of appropriate ritual. In recent years, there has been an increasing failure to attain this object, because the established cults shock their intellectual convictions and outrage their common sense. Thus their minds criticize their enthusiasm; they are unable to consummate the union of their individual souls with the universal soul as a bridegroom would be to consummate his marriage if his love were constantly reminded that its assumptions were intellectually absurd.
I resolved that my Ritual should celebrate the sublimity of the operation of universal forces without introducing disputable metaphysical theories. I would neither make nor imply any statement about nature which would not be endorsed by the most materialistic man of science. On the surface this may sound difficult; but in practice I found it perfectly simple to combine the most rigidly rational conceptions of phenomena with the most exalted and enthusiastic celebration of their sublimity."
The word "Gnostic" is a Greek word which means "pertaining to knowledge." It is often used in reference to a group of early Christian religious systems which developed in the cultural, religious and philosophical melting pot of the Eastern Roman Empire during the first century, e.v. and were centered primarily in Alexandria, Egypt. Known collectively as Gnosticism, these systems, considered "heresies" by the developing Christian theocracy, attempted to fuse Judaic Christianity with Greek Philosophy, Egyptian Theurgy and other elements.
The Gnostic current appears to have wound its way through the middle ages in several distinct streams of tradition. The Hermetic Gnosis was incorporated into the allegorical science of Alchemy. Egyptian Gnosticism influenced Hebrew Hekalot and Merkabah mysticism which later developed into Qabalism.
The union of the Gnostic Catholic Church and O.T.O. represents a confluence of numerous Gnostic streams into a single body — a body which was to achieve ultimate fruition upon the introduction of the last and most dynamic Gnostic current, the Law of Thelema, which was introduced into the O.T.O. and Gnostic Catholic Church by the Master Therion.
In composing Liber XV, Crowley attempted to uncover the hidden Gnostic tradition concealed within the ceremony of the Mass, to liberate it from bondage to the Scholastic theories and dogmas of Christian theology, and to demonstrate the fundamental continuity between this ancient tradition of Wisdom and the modern revelations and liberating philosophy of Thelema.