Religions are defining factors for theists, yet they are a matter of personal experience. In other words, a two friends of different religions can can find solace and resolve while being equally sincere in their Faith. The experience is real and tangible at the personal level. Faith provides strength of the mind when the worshipper believes to be in grace, and a weakness of mind when it believes to be impure or a sinner.
Theism and Atheism
Theism  is an advantage whereby the Will of a character is increased by 1 when the character believes to be pure or in good grace. The counter to this perk is that the belief of being impure or with unforgiven sins incurs a penalty of -1 on Will.
Theists must either select the religious [-5] or devout [-10] code of conduct that defines the rites and behaviours prescribed in order to be pure/in good grace. Devouts must abide by a much more severe code of conduct, but in return are subject to a +2 or -2 modifier on their Will.
Atheism  is an advantage that is not bound to any code of conduct. Because atheists have an equal conviction that there are no supernatural interventions over the world, their presence undermines and disturbs the conviction of theists. This abnegating effect takes place only if the atheist is known to be this way: there is no magic or invisible aura around an atheist. As a result, they benefit of a bonus of +1 on Will-based check to resist influence checks made by theists. Additionally, the presence of atheists incurs a -1 penalty to prayers if they are involved in anyway in the request for divine intervention. However, known atheists suffer -1 reaction penalty with theists.
Agnostic characters are neither theist nor atheist. They may believe in something, it just doesn't have an in-game effect.