From the "Dragon race" tread on this forum:
Tom wrote:
The dragons of Twokinds are said to be the creations of the old gods.
So gods (and the Masks, who are effectively demi-gods) are accepted to exist. They're even used in curse forms. (We've seen "by the Masks!" haven't we?) They don't seem to be worshiped much, though. It's hard to say if we just haven't seen it, if they're simply not worshiped (similar to the situation in some of Fritz Leiber's books featuring Fafner and the Grey Mouser), or if "they're not so much worshiped as blamed" (as Terry Pratchett put it for Discworld). Since the presence of a few discrete godlike beings exist, there's unlikely to be much plurality of religion, unless you're worshiping one out of the set. Now, for the old gods, of which there's no in-comic reference that I'm aware of, they might have a much more interesting religious system. However, one could infer by Tom's use of "old gods" that they are not worshiped anymore, except perhaps by small cults.
If I were to suggest a system similar to an Earthen one, I'd suggest polytheism of an ancient Greek sort. There's not any suggestion that people of various areas have different gods, so Roman-like polytheism (which adopted gods into it very easily) is relatively unlikely.