One of the interesting about the FAGE rule set is its handling of classes. There are only three: Rogue, Warrior, and Mage. Each one covers a lot of ground. Rogues might be thugs, cutpurses, scouts, burglars, and so on. Warriors could be anything from a barbarian to a chivalric knight. Mages cover all spell casters. Which means, among other things, that there isn't a separate cleric/priest class. At least not in the core rules.
This leaves me with the question of how to handle religion in the setting. Since you don't have an entire PC class required to pick deity, one could theoretically just leave it out. It seems strange to go without religions entirely, though. Gods and goddesses are such a staple in fantasy settings, to have one without them feels incomplete.
On the other hand, this aspect of the game opens up the intriguing possibility of having religion without "gods." By that I mean the people may believe in a deity or pantheon, etc. but –unlike in a D&D game where clerics of Thor call down lightning on the wicked or raise the dead– there are no overt miracles to be seen here. The faith –or faiths– power is institutional, not magical. Like the catholic church in our world, it can be a powerful force in the world, but the power is really secular, based on wealth and influence, not spell slots.
At the same time, it may well be that there are "mystics"; be they orc shamans, elven seers, human ascetics, or what have you with "the power of miracles." They may believe that their magic comes from the gods, but does it? Or are they naturally gifted mages?
The answer, of course, is a matter of faith.