Ecclesia Gnostica Catholica (E.G.C.) is the Latin for Gnostic Catholic Church, a Thelemic ecclesiastical rite that comprises the church within Ordo Templi Orientis. The chief function of EGC is the performance of the Gnostic Mass (Liber XV), a Eucharistic ritual written by Aleister Crowley in 1913. The structure of the Mass is influenced by the rite of the Russian Orthodox Church; its content draws from the symbols and texts of Thelema. Its most notable divergence from similar rites of other churches is that both a Priest and a Priestess officiate, the latter sitting unclothed on the altar during most of the ceremony. In addition to the Mass, baptism, confirmation, marriage, ordination, and last rites are offered by EGC.

Crowley described the Gnostic Mass as "the central ritual of O.T.O., both public and private". It is the single most commonly performed ritual at O.T.O. bodies, with many locations celebrating the Mass monthly or more frequently. Most O.T.O. bodies make some or all of these celebrations open to interested members of the public, so the Mass is often an outsider's first experience of the Order. E.G.C. has a hierarchical structure of clergy, assisting officers, and laity which parallels the degree structure of the O.T.O. initiatory system. Before 1997, the two systems were more loosely correlated, but since then there have been strict rules concerning minimum O.T.O. degrees required to serve in particular E.G.C. roles.