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.
Sources
- Demon: The Fallen core rule book
- Demon: Earthbound source book