Faith
Faith is what powers demonic powers, and it behaves in many ways like Glamour from Changeling. Like Torment, Faith is rated on a scale of 1 through 10, with both a permanent and temporary rating.
Faith is gained from mortals in one of two ways. The demon can either make a pact with a mortal -- the mortal provides a regular supply of faith to the demon in exchange from some task or favor. Alternately, the demon can reap the faith -- forcing a mortal to face the reality of the demon's existence, and taking the faith that generates.
Faith is used in different ways, depending on the circumstances. Some powers merely require the character to have a point of Faith available; nothing else is required. Some powers require a Faith roll in order to be activated; shifting into a character's apocalyptic form is one example. The pool of dice for this test is usually based on the character's current temporary Faith score. In these cases, the character can often spend a Faith point to automatically succeed at the test (sometimes giving the PC a point of Torment as well).
Permanent Faith is a measure of how powerful the character's invocations and other celestial powers are. It typically restricts the number of successes an invocation can generate, or limits the range of powers. It also serves as a target number for some tests. For example, mortals who see the character's apocalyptic form roll a Willpower test against the demon's permanent Faith to determine how they react.
Faith can also be used to grant celestial powers to mortal servants, called thralls. Often, this investment is the result of a pact between the demon and the mortal; for example the mortal gives the demon 2 Faith a day, and one of those points actually goes to an ability the mortal gains from the demon.
The rules regarding Faith are not terribly complex, but they are scattered throughout the book, making an easy-to-find reference for the trait difficult to manage. Fortunately, the rules for Faith in a given circumstance are generally found in the appropriate rules section for the power or interaction being referenced. Still, one question persists in my mind -- is a character's temporary Faith pool limited by their permanent score? Most of the examples given indicate this is the case, but there is no clearly defined answer that I could find.
While Faith is the source and measure of a character's power, it doesn't suffer from the problems that Arete (from Mage) does. Faith fuels the PC's powers (much like blood in Vampire), and generally determines how powerful the demon can be. It does not, however, determine the actual success or failure of power use the way Arete does. My biggest problem with the Mage system was that single dice pool governed all of the character's supernatural powers. Demon does not suffer from this problem.
Okay, so how does Reaping Faith work exactly?
Reaping Faith is easy, system-wise. The specifics of the non-system is where it gets wierd. Essentially, to Reap Faith, all you have to do is get a mortal to accept your divinity, without a doubt. Note that this does not mean that you can take Apocalyptic Form and start Reaping away. Apocalyptic Form, doesn't impart meaning, and make the point that you're a divine being, it just awes or scares the crap out of the mortal in question. The mortal has to truly believe the demon truly is a messenger from Heaven or creature from Hell.
So, then, how does this work? For Low-Torment demons, we're talking about acts of kindness and heroism; saving a jogger from a rapist by appearing in Low-Torment Apocalyptic Form, for example. High-Torment demons have more fun...appearing as a monstrous thing and flaying a victim with it's claws is popular. The thing that matters is that the mortal truly believes that they are interacting with a Demon. You don't do that by taking Apocalyptic Form and saying "I'm a Demon."
Once the divinity is beyond question, then the easy part comes in. It's a contested WP roll between the target and the Demon, both at 7. If the Demon gets more successes, he gets a Faith point. In some cases, with an knowledgable ST present, the Demon may qualify for 2 points, IF the Demon's actions resonate strongly with the mortal's personality, beliefs, or personal history. This is exceptionally rare, and won't be given out commonly, but it can happen.
Faith
Faith is the divine spark, the source of the Fallen's power - with sufficient Faith they really can uproot mountains, but without Faith they become powerless spirits.
Before the rebellion, all Elohim received their Faith from the Creator, but once the rebels were cast out, they could no longer access that source. However, the divine spark had been granted to the pinnacle of Creation - humanity. After casting their lot with humanity, the Fallen were able to tap into this potential and confront the loyalist Elohim on an even footing. This relationship did not endure, however. When Lucifer attempted to force humanity's development and failed, the human soul shattered and the link between the Fallen and humanity was severed. The Fallen's reserves quickly ran out, and they were rounded up and cast into the Abyss.
After their return to Creation, the Fallen have since found out that though the link between themselves and humanity is tenuous at best, it is still sufficiently strong to harvest Faith from humanity. The Fallen can do this in several ways:
Reaping Faith: By confronting individual humans with either heavenly benevolence or infernal malice and cruelty, the Fallen can tap people's souls for a small amount of power.
Donating Faith: People who have forged a pact with a Fallen may donate some Faith to their masters every morning to fulfill their part of the bargain. This influx of Faith is slow but steady, and can become very useful if the thralls are spread all over the globe, as each thrall delivers up their Faith at each subjective sunrise.
Worship: Earthbound lack the deep empathy with humanity the Fallen enjoy, and thus can only gain Faith through concerted worship and ritual. Each ritual can supply the Earthbound with a substantial amount of Faith, but there is a limit to how many rituals can be enacted each month. Only the most powerful Earthbound with globally operating cults can count on receiving Faith in this way every day.
Ravaging Faith: Fallen can rip more Faith from the souls of those who already donate Faith than was originally agreed upon. This act causes the thralls discomfort and pain, and may even physically injure or kill them. There is no limit to the number of times a thrall may be ravaged for Faith, but if the Fallen wants to keep the thrall alive and happy, this option should only be employed in an emergency. Ravaged Faith does not end up in the Fallen's Faith pool, but it will greatly increase the power of an evocation by providing extra dice for the effect roll. Earthbound cannot ravage their thralls for Faith, but they may have their followers enact special rituals that serve a similar purpose.