We are not the tyrants, rather, we were made to cast them down, those that would hold humanity in ignorance, as slave to false gods. The Astartes willingly make this sacrifice for all. And no, the Emperor's decree is that there are no gods, that is the Imperial truth. Just the lost superstitions and half-understood truths of the warp. We are a secular and rational society, not one blinded by zealotry.