The soul of Mankind is bound to the Emperor not by unconditional reverence, but by the rule of the Imperial Creed. Though it takes as many specific forms as there are planets or even cultures in the Imperium, the one, true faith unites every man, woman, and child, every scribe, worker, and soldier, every noble, planetary governor, and High Lord. Those who would deny their duty—to the Imperium, to the Emperor, or simply to the overseer of their workplace—are deemed “heretic,” the enemy within, and cast out from the beneficence of the Emperor.
Heresy takes a myriad of forms, often varying drastically from one culture to another. Expressions of faith made on one world might be condemned as blasphemy on another, and while interplanetary travel is rare, when strangers from such divergent cultures do meet, conflict is not uncommon. When worlds are isolated for prolonged periods, the doctrines of faith are wont to diverge and mutate. Sects rise, and formerly proscribed cults step out from the shadows and preach openly to the masses. Such blasphemy against the Master of Mankind can lead ultimately only to the worship of other, darker entities: Chaos.
Those who turn to Chaos are forever damned, and in their damnation they endanger all of humanity. Some believe themselves to be sowing seeds of purity and truth as they denounce the hated corpse-god of Mankind. Others are warriors who find in the service of the Ruinous Powers the means to unleash undreamed-of destruction on their enemies. Most terrible of all those who fight for the glory of the Ruinous Powers are the Traitor Astartes. Once ranked amongst the mightiest of the Emperor’s champions, these traitors turned against their master and creator ten thousand years ago and continue their Long War even into the current days of the Time of Ending.
The more outwardly visible expression of the taint of heresy is that of mutation and deviation from the divine form of Man. While the Imperium tolerates certain genetic divergence—sub-species such as the brutish Ogryns or diminutive Ratlingss, and mild mutations to be endured within the depths of a hive factory or on fringe worlds—these exceptions are rare. Most mutants are hideous creatures tainted in both body and soul, bearing distended limbs or oversized appendages, tentacles, claws, or countless other hideous signs of some unknown sin. Some worlds use the more easily controlled mutants as expendable slave labour, while others exterminate them wholesale in fiery pogroms and wars of faith.
Many mutants display their heresy in bodily form, but the more insidious mutants possess mental aberrations, and are decried as witch, sorcerer, Warp-dabbler, or, more commonly, Psyker (DHII). Each year, ever more psykers are born or come into their powers, but the vast majority are entirely too weak to control their abilities. If allowed to exist unchecked, they bring down death and destruction on their communities, as some can slay with a mere glance or blast sheets of flames from an outstretched hand. Worse, psykers cannot help but attract the attentions of the denizens of the the Warp, for their powers are fuelled by a special connection to that unreal dimension. Unable to fend off such unholy predators, the psyker
falls victim to possession, and his mind can become a gateway through which the Daemon of the Warp burst forth into reality. Such daemonic incursions all too often lead to the death of entire worlds and even systems.