Though Shawn's explanation is accurate , it doesn't make sense in a world where deity faith is specific. Now mind you this might be intended for this world, and the devs have their own reasons which shouldn't be questioned, especially after being answered.
Now saying that having a cleric and paladin use faith in a deity it doesn't know exists is wild. There wouldn't be a point to the faith of the unknown. The mere thought would put them in doubt of their own abilities
