The Imperial Truth was the founding creed of the Emperor's new order: the conviction that there were no gods, that the supernatural was superstition, and that mankind's future lay in reason, science, and secular unity. It was the philosophy that justified the Great Crusade, sweeping away the cults and faiths of the worlds it reconquered in favour of a rational, godless enlightenment.
The irony of the Imperial Truth was that it was, in part, a noble lie. The Emperor knew well that the warp was real and its denizens hungry, but He judged that to name them gods would be to feed them, and that humanity's best defence lay in denying them worship. To this end He forbade religion even as He waged secret war against the powers of the empyrean.
The creed could not hold. The very primarchs it was meant to guide encountered proof of the warp's horrors, and the traitors used the Truth's denials against it, offering the certainty of gods to those disillusioned by the Emperor's silence. Worst of all, in defiance of the Truth, some began to worship the Emperor Himself. By the war's end the great secular dream was shattered, and the Imperium that survived would raise its founder as the very kind of god He had forbidden.