Board Thread:Warhammer 40k General Discussion/@comment-9558536-20130517084538/@comment-4264245-20130517092512

You should read (or re-read) Galaxy in Flames and how Euphrati Keeler becomes a Saint. The Emperor had an extreme amount of power - he was all-knowing, ageless, absolutely inspiring and able to change reality in almost any way with just a clack of his fingers. That made him unable to understand common people's fears about the unknown, the death and the impossibility of things - therefore, their need of a faith in a superior being who could overcome all of them, and it also made him the perfect object of worship as a living god who walks amongst mortal humans and protects them from an hostile galaxy.

When the Emperor sacrificed his body to stop Horus from conquering the Imperium and was interred in the Golden Throne so that he could keep shielding Terra and his servants with his mind's power alone, more and more people became convinced that, if he hadn't been a god before, now he truly had ascended to divinity, and the groups formed around Lorgar's Lectio Divinitatus (ironic, huh?) gained influence over society and then outright power. The recognition of the Ecclesiarch as a High Lord of Terra was just the acceptation of the de facto situation, and it also served to give a common bond for the distraught and disparate worlds of the Imperium after the Horus Heresy.

After that, ten thousand years passed, everyone save the Adeptus Astartes forgot the Emperor's wishes (most likely they were hid by the Ecclesiarchy) and now if you dare to say He-on-Terra is not divine, you burn slowly and painfully xD