I'm not arguing that Shadow Priests don't have faith in anything, or that it isn't strong, but I think their faith is of a somewhat different nature. Shadow Priests certainly don't trust the Void. Even Natalie Seline, who was said to be obsessed with its study, believes everyone should be worried by the whispers of the Void.
So, if Shadow Priests recognize the Void as a danger and threat, with plenty of warnings against being too trusting of it or Void entities, what exactly do they put their faith in?
In the RPG, the faith of a Shadow Priest was in the strength of their own will and ability to wield the power they possessed with proper care.