Proto creed that would eventually become the Cult of the Emperor. Saw it's fastest spread during the Horus Heresy. Under this creed the Emperor of Mankind was seen as a God-Emperor. The Lectitio Divinitatus is a direct threat to the Imperial Truth. (Mythology of Warhammer 40,000, originally just a miniatures war game, now encompassing miniatures, cvg's and multiple series' of novels by many authors)
"The faith of the Lectitio Divinitatus, the Divine Word of the Emperor, is not an easy path to follow. Where the Imperial Truth is comforting in its rigorous rejection of the unseen and unknown, the Divine Word the strength to believe in that which we cannot see. The longer we look upon this dark galaxy and live through the fires of its conquest, the more we realise that the Emperor's Divinity is the only truth that CAN exist.
(Ben Counter, 'Galaxy in Flames', 2006, pp17)