The Imperial Iterators
An Iterator is an Imperial rhetorician, propagandist and educator who is a part of the Imperial Corps of Iterators created by the Emperor of Mankind to accompany every Expeditionary Fleet of the Great Crusade.
The Corps of Iterators were created by the Emperor specifically to spread and reinforce the atheistic and rationalist Imperial Truth among all the worlds conquered by the Great Crusade and brought into Imperial Compliance. Iterators are present on every Imperial world where they serve as educators and propagandists who seeks to insinuate the Imperial Truth into every aspect of human life. Iterators pays particular attention to the eradication of any religious or superstitious beliefs among the populace, replacing them with calm, cool rationalism and a strong, almost Victorian belief in scientific progress.
None better than an Iterator can explain the advantages brought by Imperial civilisation or the reasons why the conquest of tens of thousands of new worlds by the Imperium is a necessary requirement for human advancement despite the damage caused to existing human cultures.