The Hierocracy was a time when the Vorin Church attempted to rule on Roshar politically rather than merely acting as a spiritual guide. The Church tried to conquer the world and control the people "for their own good." Priests controlled what people learned, what religious paths they followed, and the overall doctrine, because they claimed that the common people could not understand theology. They justified this with visions and prophecy they claimed to see.
”They were taught to follow the priests. Not the Almighty or the Heralds, but the priests.— Kadash