The faithful button will allow you to go against your gods wishes, permitting you to perform acts that your deity would otherwise not allow, such as Magranon and Vynora’s dislike of butchering human (or trader) corpses.
Losses for being unfaithful:
Butchering human corpse; -0.3 faith, -5 alignment
Destroying Altar of Three; -20 faith, -50 alignment
Success in destroying altar of your God; Championhood revoked, thrown from religion
Loss of priesthood: When you go below 30 faith, you will lose your priesthood, and your faith will be set down to 20.
(please add more)