Uriah Olathaire was the last priest on Terra from before the Imperium of Man was founded. He was confronted by The Emperor before his church was destroyed. He is the subject of the Podcast episode The Last Church, based on the short story of the same name.
Uriah was the priest of the Church of the Lightning Stone, the last church on Terra. He was visited by a man who called himself "Revelation," who proceeded to explain why his congregation left and why the Emperor's Imperial Truth was going to replace all religion. Though Uriah eventually broke down and lost his faith, he was also humiliated by the Emperor's Thunder Warriors by being thrown off a cliff and having his church set on fire. Uriah threw himself into the flames, unable to accept the new future.
Upon discovering the Black Library book containing The Last Church story, the Emperor decided to call up Uriah's soul from the Warp to have their debate once again. However, all present were horrified to discover that the priest had converted to Chaos worship since he died. Uriah pointed out the various hypocrisies of the Emperor's Imperial Truth, both during the Great Crusade and after, and how Chaos is the only true path for humanity. While the Emperor's arguments failed to convince Uriah, it wasn't until he mentioned how he kidnapped Magnus the Red and stole back his soul from Tzeentch by stabbing him repeatedly in the face that he started to express doubt. Nonetheless, because their debate had been on a live Vox broadcast, the Custodes reported that some new civil wars had started up. Uriah was eventually banished from the room for claiming that tacos weren't that great, much to the disgust of all present.