Despite His best efforts to promote peace and harmony across Old Earth, the instinctive values of martial honour, ambition, defiance, and self-satisfaction could never be eradicated from the Human character. With the final abatement of the Warp Storms caused by the birth-pangs of the Chaos God Slaanesh and ended by the Fall of the Aeldari, the Emperor finally began the Great Crusade in ca. The objective was a Great Crusade that would unify all of the planets colonised by Humanity during the Age of Technology prior to the Age of Strife into one Imperium of Man.

At the same time, the alliance formalised the creation of the Imperium of Man and established the Imperial bureaucracy on Terra, integrating the Mechanicum as one of the myriad organisations that comprised the newborn Adeptus Terra, the massive government of the Imperium, the future Priesthood of Earth.The primarchs were sucked into the Warp through the will of the Chaos Gods while still in their gestation capsules and scattered across the Human-inhabited worlds of the galaxy. Few could have managed to examine them in such a fascinating light either, or given them the sense of grandeur they required to press home their importance.

So, they believe that Master of Mankind helpfully strengthens their beliefs, while everyone else who believes the Imperium has a chance (albeit, small) are incredibly annoyed by this position, which has led to arguments between the two camps.

One thing that stood out was how ADB was able to get me acquainted with new characters as quickly as they were introduced. We wouldn't have Fulgrim's level of self-indulgence and autonomy (not to mention the Emperor would have had to check in on his 'weapons' more often and he'd spot the Blade of the Laer in a heartbeat) and we certainly wouldn't have had the Emperor give up the Great Crusade to Horus the way he did. As more and more Humans were born with the mutant psyker genes that granted them the ability to wield the potent power of the Immaterium in the last centuries of the Dark Age of Technology, and Humanity suffered from the deadly effects of uncontrolled psykers that heralded the onset of the Age of Strife, the Emperor realised that He would have to take a more direct and open role in Human affairs than ever before.