True Believer (3.5e Trait)
From D&D Wiki
You have a habit of evangelizing to most people you meet.
Prerequisite: You worship a deity.
Benefit: You gain a +2 bonus on Knowledge (Religion) checks.
Drawback: You take a -1 penalty on Diplomacy and Bluff checks when dealing with people who worship a deity different to yours.
Roleplaying Ideas: Perhaps a character with this trait is very defensive about their religion, or harbors a grudge against the religion of those who persecuted them.