The two documents published together as The Great Secret were in all likelihood the final doctrinal work composed by the nineteenth-century French adept Alphonse Louis Constant, better known as Eliphas Levi. They were written for manuscript circulation among Levi's pupils, and published only posthumously. Another prior section that makes of these two a set of three was published as The Book of Splendours. However, the content of that volume (on "The Hieratic Mystery") is not strongly connected to the issues in this one (on "The Royal Mystery" and "The Sacerdotal Mystery"), and The Great Secret can be read to full benefit without prior reading of The Book of Splendours, which consists of Levi's glosses of qabalistic and Masonic doctrines, along with cross-cultural comparisons of the Christ myth.

"Let it be well understood that we are not writing for the profane masses, but for the instructed of a later age than ours and for the pontiffs of the future." (105)

Much of The Great Secret addresses theological issues, and in a most bewildering manner. Levi, who had at one point trained for the priesthood, consistently professes himself to be a loyal Catholic, and to champion the Roman Church as the sole legitimate repository of magical power. At the same time, he relentlessly criticizes both doctrines and practices of the Church in his day, often proposing alternatives informed by advances in materialist science as well as comparisons with non-Abrahamic religions. Levi's occult terminology is generally straight out of the Mesmerist milieu of the early nineteenth century, with a great deal of attention given to "magnetism" in its various manifestations.

A highlight is the chapter on "The Magnetism of Evil," where Levi spends a lot more time describing and illustrating the likely (extra-)moral consequences of an objective view of the natural world than he does contradicting them with an orthodox theology to assert the solicitude for humanity by a sovereign ruler of the Universe. (In Jason Colavito's blog, he has supplied a fresh translation of this chapter, noting its presentiments of both Lovecraftian cosmicism and "ancient astronaut" Nephiliphilia.)

"The present writer is a Catholic of the desert. However, there is nothing frightful about the Thebaid, and he has always preferred the Abbey of Thelema, founded by Rabelais, to the Hermitage of Saint Anthony." (173)

The ante-penultimate chapter is itself titled "The Great Secret," and the arcanum is one well-circulated among Thelemites today. "There is no part of me that is not of the gods" (Papyrus of Ani via Liber XV). Deus est homo. "I am clothed with the body of flesh; I am one with the Eternal and Omnipotent God" (Liber LXV I:53). There is no god but man. Levi reached this conclusion in parallel to Ludwig Feuerbach in the same window of European history. Although they wrote for vastly different audiences, both men had difficulty making themselves understood. As Aleister Crowley would later remark, "An indicible arcanum is an arcanum that cannot be revealed."