True faith wasn't explained in the nWoD.
As for Revised (VTM) version, here it is :
True Faith is a special Trait that only a few people in the World of Darkness possess. While many mortals are more or less devoted to a belief in some form of higher being or purpose, only a small number have the burning zeal, the profound conviction that can protect them against creatures like vampires. Note that, while most vampires legends potray the Damned being repelled by crosses and the like, Faith can manifest in any religious form. A devout Jew might be able to ward off vampires with her Star of David, or a Taoist might be able to intone special prayers, while a Christian lacking True Faith finds his crucifix impotent against the undead.
This Faith is not necessarily increased through experience. Certainly, it may rise as a result of a person's experience, but it is more vital than that, more a measure of conviction and strength of mind. Nor is it something that comes from outside the individual, from some God or angel. Whether or not their beliefs are correct, these people believe so fervently that their own conviction protects them. At the Storytellers discretion, Faith should rise or fall to reflect a person's religious certainty and zeal.
The Faith Trait, like any other, has a rating of 1 to 5. Exactly what protection is afforded to the individual by the Trait depends on its rating, as described below. Of course, these are just guidelines. Storytellers should amend them as required to fit the tone or add drama to their stories.
• Any character with Faith may attempt to ward off vampires by brandishing a holy symbol or uttering prayers. (This is the Hollywood cliché of the vampire being held back by a crucifix) The person rolls Faith against a difficulty equal to the vampires Willpower. The number of successes indicates the number of steps backward the vampires is forced to take. If no successes are scored, the vampire need not step back but may not advance. A Botch indicates that the vampire may advance unhindered. Further, if the cross, Bible or other symbol is placed against the vampire's body, each successes causes an aggravated health level of damage burning into the flesh.
•• A mortal with a Faith rating of 2 or more mat resist vampiric Dominate by spending Willpower (one point typically protects for a few turns.)
••• A person with a Faith of 3 or more mat sense the presence of a vampire. She need not consciously try to sense a vampire's presence, but must be in peaceful, quiet surroundings - perhaps alone in though, praying, reading Torah, meditating on the Bible, Etc. The person will not sense the vampire's presence if she is preoccupied (e.g., arguing) or in a crowded, noisy place (jostled by a mob, in the midst of a raucous banquet, etc.) This ability need not be infallible; the Storyteller should let the person sense the vampire only when is is dramatically convenient. Note that the person cannot know exactly what she senses through her Faith; all that she will know is that something unclean or evil is nearby.
•••• The mortal may not be turned into a ghoul and is unaffected by any mind-altering Disciplines like Presence and Obfuscate.
••••• The person is so pure, so holy, that she can fill a vampire with self-loathing, disgust, terror, even physical pain. Ant vampire hearing the person pray, preach or recite psalms, or being touched by the person, may be forced to flee immediately by any availble means. A vampire who is unable to flee is reduced to a gibbering wreck, flailing on the floor screaming, sobbing or begging for forgiveness. To avoid fleeing, the vampire must either expend one Willpower point per turn or must make a Stamina roll each turn (difficulty of 5 + her own Intelligence). Thats right --- the higher the vampire's Intelligence, the higher the difficulty, because the more guilty she will feel.
In theory, a mortal might have a Faith rating higher greater than 5, but these people are one in a billion - the sort of people who are venerated as saints. They are unlikely to enter a chronicle (and certainly no more than once), but their power would be enormous. In vampiric terms, holy Methuselahs. In fact, some Clan Antediluvians might possess this Trait, in spite of the fact that they are vampires - Saulot and Cappadocius.
There is actually one easy way for a vampire to detect a person with Faith rating, other then bitter experience - it is always (painfully) visible when using Aura Perception. A mortal with Faith has her aura permanently altered so that she has a silver/gold "halo" around her body. The strength of the Faith determines the brightness of the halo. Note that the halo is not normally visible; only the use of Auspex reveals it.
Remember that Faith represents a person's total commitment to her beliefs. That person will act accordingly. Those with high Faith ratings may seem fanatical, even insane to those not of their religion.
Note that Cainites will not normally have Faith ratings. They consider themselves the Damned, after all. Think very, very carefully before letting any vampire have this power.
(thank you WW wiki : http://wiki.white-wolf.com/worldofdarkness/index.php?title=True_Faith)
I voted "no true faith at all" because I think it's a very very odd merit that should only be available to a few dozen NPCs around the world. It should be a unique ability possessed by those persons, and not a wide system usable by players.
It's like the fame trait over 5. Yes, Elvis and Jesus have a higher fame, but players are unlikely to cross either, and they will not play any of them.
I think, however, that kindred (and ghouls, to a certain extend) should be afraid or suspicious around faithful persons, but that is more of a psychological reaction rather than a true effect. Mass hysteria is awesome
For Hunter the Reckoning, I will say nothing, I may become very very rude
(with Demon, probably one of the biggest mistakes of the oWoD)