True Believer (3.5e Trait)
From D&D Wiki
You have gone through hard times because of your religious background, but you persevered. Because of this, you're mentally tougher but less of a people person.
Benefit: +1 to will saves, +2 Knowledge (Religion)
Drawback: -2 to interaction rolls when its not directly for religious cause.
Roleplay: Perhaps a character with this trait is very defensive about their religion, or harbors a grudge against the religion of those who persecuted him/her.