It's something to consider from the inverse of your question at any rate.
For myself, I figure that they used analogues of the real world to construct Thedas. Many things are loosely based in some way on the medieval world. And in fantasy, there is always some kind of mythology. In this world, they have a monotheistic Maker mythology that fuels the dominant religion of Thedas. You also have the polytheistic older religious mythology of the Dalish. No one is sure exactly which of the two mythologies are correct, or maybe both, but it's not a stretch that there must be some truth there given the very real demons and spirits that you have to fight against.
It's not like this is any different than picking up any one of a hundred different fantasy novels and accepting that the world has a mythology which only serves to deepen and make the place richer in the end.
Of course, I guess an interesting question would be to ask how many of you have read any of Jacqueline Carey's Kushiel series? If you have, did the religious mythology of Terre d'Ange bother any of you who are atheist?