Tower and Tomb
Taking on the mantle of a faith, of any faith, is not easy. It involves giving up your own views, at least in part, in exchange for the vision revealed by scripture, teaching, illumination, or even experience. Doing so is only slightly easier in the context of a roleplaying game.
The important thing to remember, especially for religious belief in the Dark Ages, is that faith is real. People of medieval Europe knew for a fact that there was a God, that humans had souls and that something would happen to them after death. This knowledge was no more questioned, or questionable, than the existence of the earth, rocks, air, or water. In fact, it was questioned less often than the existence of those things. Someone denying the spiritual realm was simply talking nonsense. Demonic possession was the kindest theory one’s neighbors could construct to explain such blasphemous behavior.
Now, the specific nature of the common faith is what all those Crusades and witch-burnings and arguments and riots and students being dunked in the River Seine were about. You needn’t have a complete grasp of the Doctors of the Church or the difference between homoiousios, homoousios and homoios to play a medieval Christian, or even to be a medieval Christian in most cases. (But if a Byzantine torturer asks you, choose the middle one, otherwise we can’t be held responsible for the consequences.) Even priests often knew the words to the Mass but not their meaning and repeated the liturgy like a magical chant rather than explicating the mysteries to their flocks.
Needless to say, many folk grew cynical about their Churches at such a time—that dissatisfaction is, after all, part of why heresies spread so far and fast during times of widespread ecclesiastic corruption. Another part of the equation, however, is the fact that virtually all of the populace felt a great need to be connected, often as closely as possible, to the world of the numinous and the holy. A sermon preached with fiery zeal in their home dialect by a mad-eyed stranger touches villagers who have only heard the Mass droned in a language they don’t understand. They have been fed, spiritually.