Origin is explained in the Book of Origin, carried by all who believe in the faith. Priors are dispatched to regions in the cosmos where there are nonbelievers and those who have never-before encountered the belief system. Eventually they are either converted or destroyed.
Avalon, Part 2 - Daniel and Vala begin to learn about Origin and the consequences for not following in its teachings.
Origin - A Prior takes Daniel and Vala to the city of Celestis, where Daniel is granted an audience with the Doci who explains Origin and its doctrine.