To become a true believer, you must have the A template from each of the three disciples, and be in good standing with the faith.
The True Believer gets +1 to save, attack, and defense, and +2 FD.
A. Blessing of True Belief
So devoted to the Family, your actions are no longer on behalf of the Family, but of the Family. You are unable to perform actions that are opposed to the will of the Family, but in exchange, your faith dice are now permanently d6s, and you gain 1 extra spell of your choice from each list (1-10).