The Lectitio Divinitatus (also Lectio Divinitatus) was a book written by the Primarch Lorgar during the Great Crusade, before his fall to Chaos. It postulated the worship of the Emperor of Mankind as a divine being. The following of the Lectitio Divinitatus became an underground cult, believed to be the precursor of the Temple of the Saviour Emperor and the Imperial Cult, the basis of the modern Adeptus Ministorum. During the Great Crusade such cults were frowned upon by the Emperor and the Council of Terra, and most especially by the Astartes and those serving them. The Emperor downplayed his power but those who believed in the Emperor's godhood were of the opinion - as Titus Cassar once put it - that "only the truly divine deny their divinity".[Needs Citation]
In the 63rd Expeditionary Fleet, commanded by Warmaster Horus himself, the cult was most prevalent. Horus took steps to try to extinguish the cult, as he perceived it as a threat to his power. The remembrancer Euphrati Keeler - who was attached to the 63rd - became a major figure, venerated as a living saint and prophet of the Emperor. As a result, she was the target of Horus' assassins at some point before she escaped from the Vengeful Spirit.[Needs Citation] Even some Astartes - such as Nathaniel Garro, the Death Guard loyalist who led the Eisenstein escape - became part of the cult at one point.
At some point after the Emperor was interred in the Golden Throne, it may have been one of the cults that took part in the war that eventually saw the rise of the Church of the Saviour Emperor, a rival, rise to power as The Ecclesiarchy 
- 1: Scions of the Storm (Short Story) by Anthony Reynolds
- 2: Horus Rising (Novel) by Dan Abnett, Part One, Chapter Three, pgs. 58-59
- 3: The Flight of the Eisenstein (Novel) by James Swallow, Chapter Fourteen, pgs. 344-345
- 4: Codex: Sisters of Battle (2nd Edition), pg. 5