I think the answer would be that any particular Frank could be any religion, including a modern real world religion like Christianity, if you really want to get into that in the game (personally, I would rather not). I do not believe that actual medieval Christians were constantly and unremittingly hostile toward every member of every other religion that they encountered under any circumstances (or at least not when outnumbered and unable to convert "pagans" by the sword, which would be the case in Hyperborea), so I don't see why that would be required in the game. I don't feel comfortable revealing even minor details of the play test document that Jeff sent to some of us, but it's probably safe to say (given how fraught the issue would be) that the Carolingian Franks as a whole are not Christians in Hyperborea.

Ghul wrote:

CAROLINGIAN FRANK
A race of medium to large people with fair skin, and variable hair and eye colour. In days of yore, Carolingian Franks dwelled in the Cape Calencia region, but they were nearly decimated by the Green Death. Their race is descended from warriors who served Charlemagne of Old Earth. They were at war with the Moors, whom they engaged in the Pyrenees Mountains. When the opposing legions met in a fertile mountain valley farmed by peasants, a strange being known as Tantugou engulfed both armies in a churning mist and whisked them away to far-flung Hyperborea. Most Carolingians have returned to the faith of their pagan roots, replacing Christ with Apollo. Typical homelands are as follows:
 

• Cape Calencia: A small, forested horn with massive redwood trees. In small family groups, Carolingians build on the ancient ruins of Hyperborean settlements.
• City-State of Khromarium (see “common” race)

Well that covers everything I was looking for. Thank you!