True faith is the genuine, preferably fanatical, belief in a (nearly always religious) ideology. While many may claim to be of a faith, generally very few are considered to have the serious, genuine devotion to count as true faith, especially in the modern world. Since the power is derived from the faith itself rather than connection to a deity, this faith need not be to Christianity or even an Abrahamic faith, and even "religions" that are provably scams can have members with true faith. In /tg/ context this is largely important for repelling supernatural creatures, most famously Vampires, that are weak to it.
World of Darkness
Characters with True Faith can repel vampires, wraiths, and demons as you'd expect. It can also disrupt magic, and revert Werewolves. Those with six dots and higher (the level disciplines explicitly become plot device powers) can perform miracles, up to and including reversing the Embrace. Often the only reason such characters aren't literal saints is they aren't dead yet.
The Society of Leopold (which is really the Inquisition) has the most prominent users of faith in the modern world, with the Celestial Chorus a follow up, but many hunters and even some other supernaturals possess it as well. Werewolves have an interesting relation with true faith because of the talking to spirits thing. There's even a few vampires with true faith.
True Faith is, mechanically, a purchasable trait like many others.
Monster Hunter International
Faith can repeal vampires, even masters. So far this and operation of a Saint Hubert's Key (a relatively common Catholic monster detector artifact) is the only known use of faith. True faith is most common in religiously motivated hunting organizations like the Catholic Church's Blessed Order of St. Hubert the Protector (AKA the Secret Guard), but MHI itself likes to keep at least one person with true faith on each team of monster hunters, implying it's not that rare. Even the cultists of an evil deity can repeal vampires with their faith.
Mechanically faith is a learned skill in the Hero version, which is then used to oppose the vampire's ego score. In the Savage Worlds version it's a Spirit contest, with no other requirements. This version is a bit wonky since Spirit is an attribute all characters have, making it a bit too common.
Dungeons & Dragons
Faith itself is what powers divine magic in Eberron. As a side effect, there are no alignment restrictions for non-Paladin divine casters and most of Eberron's religions have a variety of splinter faiths.
In GURPS it's a Supernatural Advantage costing 15 points and repels demons and undead up to 5 yards. An Enhancement for a few extra points makes it into basically a D&D cleric's Turn Undead feature. One of the sample characters for 4th Edition is a killer robot who is a Buddhist monk with True Faith.