The Lectitio Divinitatus (also sometimes spelled Lectio Divinitatus) was a religious book penned by the Primarch Lorgar of the Word Bearers Space Marine Legion during the Great Crusade of the late 30th Millennium, before his fall to Chaos. The book, and later the religious movement that it spawned, postulated the worship of the Emperor of Mankind as a divine being, the one, true God of humanity.
The following of the Lectitio Divinitatus also eventually became an underground religious cult of the same name across Imperial space by the end of the 30th Millennium. This cult is believed by current Imperial savants to be the precursor of the Temple of the Saviour Emperor and the Imperial Cult, the precursors of the modern Ecclesiarchy and the state religion fervently practiced in the late 41st Millennium by trillions of human beings across the galaxy. During the Great Crusade, such cults were deeply frowned upon and condemned by the Emperor and the Council of Terra, and most especially by the Legiones Astartes and those serving them, as a violation of the secular Imperial Truth which held that only reason and science offered a guide to understanding the universe. The Emperor denied his divinity outright (a somewhat ironic posture considering He had created an entire host of Earth's ancient religions in His various guises over the centuries of human history), but those who believed most firmly in the Emperor's divinity were of the opinion -- as the officer of the Battle Titan Dies Irae, Titus Cassar, once put it -- that "only the truly divine deny their divinity."
In the 63rd Expeditionary Fleet, commanded by the Warmaster Horus himself, the Lectitio Divinitatus cult was very prevalent, though hidden, among a large minority of the men and women serving in the fleet. Horus took steps to try to extinguish the cult, as he perceived it as a threat to his own power, particularly after he embraced the service of Chaos following his healing at the Temple of the Serpent Lodge on the world of Davin.
The Remembrancer Euphrati Keeler -- who was attached to the 63rd Expeditionary Fleet -- became a major figure of the early Lectitio Divinitatus cult and one of the first Imperial Saints after the establishment of the Ecclesiarchy in the 32nd Millennium. Keeler was venerated as a prophet and living saint of the Emperor even during her own lifetime. As a result, she was the target of Horus' assassins at some point before she escaped from his flagship, the Vengeful Spirit, at the bidding of Captain Garviel Loken and with the aid of the Luna Wolves Captain Iacton Qruze. Even some Astartes -- such as Captain Nathaniel Garro, the Death Guard Loyalist who led the escape of the Loyalist frigate Eisenstein from the massacres on the world of Istvaan III in the Istvaan System -- later became part of the cult, largely because of Keeler's influence and their own experiences with how faith in the Emperor could be used as a weapon against the supernatural horrors of the Warp.
At some point after the Emperor was interred in the Golden Throne following the end of the Horus Heresy in the early 31st Millennium, billions of worshippers emerged from all across the Imperium. This faith unified the Imperial people against the myriad threats and horrors of the galaxy in their common belief that "faith in the Emperor protects" until the cult of the Lectitio Divinitatus became the official state religion of the Imperium in the 32nd Millennium. As such, the faith in the God-Emperor inspired by the Lectitio Divinitatus became the foundation of the Imperial Cult administered by the Adeptus Ministorum.