Imperial Truth
The Imperial Truth is a doctrine espoused by the Emperor of Mankind as part of the Great Crusade. According to the Imperial Truth, the Emperor believed that religion and superstition had divided mankind too much during its history (which the Emperor had reason to believe, having lived through most of Terra's religious wars). When the Great Crusade left Terra, the Imperium of Man was to become a secular state espousing science and reason. Following the Horus Heresy and the Emperor's internment on the Golden Throne, the Imperial Truth collapsed and the Imperium of Man has become a theocratic dictatorship where questioning anything results in a painful death.
However, the Emperor didn't just ban religion simply because he disagreed with it. During his millennia of existence, the Emperor had come to partially determine the nature of the relationship sentient life had with the Warp. Knowing that the belief in gods would only serve to empower the Warp creatures, the Emperor hoped to starve the Chaos Gods by denying them worshipers. What the Emperor failed to understand was that the Chaos Gods were powered not by faith, but by emotions. People going about their daily lives experience their normal emotions would still empower the Chaos Gods. What was needed was an alternative to direct the belief at, such as a God-Emperor.
An alternative view is that the Emprah's plan wasn't to actual kill the Chaos Gods but to remove them from the Materium. The Emprah had a few major pet-peeves: religion, uncontrolled psykers, xenos and warp travel. Without cultist-chan and chaos worshiping xenos, there would be no summoning of demons. Without uncontrolled psykers, few possessions would occur. Without warp travel, there would be few opportunities for Chaos Gods to fuck Man up. Thus, without religion, xenos, psykers and warp travel, the Chaos Gods and their minions wouldn't be able to access the Materium (for the most part). Who gives a shit if the Chaos Gods are powerful if they are unable to do shit with that power? Answer, Emprah knows BEST!!!!
Anyhow, when Horus had a vision of the 41st millennium, part of his instant parricidal rage was that the Emperor was abandoning the Imperial Truth by seeking to become a real god. This in turn convinced him to renounce the Emperor. And then start worshiping the Chaos Gods. Chaos is a funny thing.