What is faith? Dictionaries may be interpreted to tell us that the definition is steadfast belief without or in spite of evidence. This is a fairly modern definition, and, of course, it gets rendered meaningless in any game system where faith produces actual, generally magical, effects. So I ask again, what is faith? The answer is manifold.

Faith is science. It is a way of defining and explaining the universe. Why does the wind blow? What happens when we die? Where did it all begin? The questions are the same, but the answers are different. In our world, science can generally provide proof for its answers, and when it is proven wrong it will adjust itself. Faith doesn't, but in worlds where faith does, where its explanations are backed by proof, then it is science. The paradigms of reality are simply shifted.

Faith is belief. Where science is arguably universal, faith works, in part, because you believe. Its almost a perpetual motion machine where strong belief lets you see, rightly or wrongly, the miracles of faith in everything. Without this belief, it is ordinary, if it occurs at all.

Faith is community. The proponents of a faith find greater strength among each other. They feed upon one another's energy, and bolster their world views. Like any social organization, when a faith attends to its community, it thrives. When it does not, it dwindles. I look at the church I grew up with, St. Monica's on Mercer Island. It used to be a vibrant community with all sorts of activities from rummage sales to Easter egg hunts to Irish coffee and dancing. Now it is a pale skeleton if its former self and the only community activities seem to be lectures on the faith.

Faith is personal. You are touched by the faith, and it is your belief, your conviction, that fuels your connection with whatever you hold to be divine. This is not something that anyone can grant you or strip from you unless you let them.

Faith is distinct. Now matter how similar two faiths are, they still have differences that set them apart. These differences manifest very often in religious dogma. They also manifest in the beliefs of the faith. The belief that the divine is vengeful creates a difference from a faith where the divine is forgiving.

Faith is not religion. Religion is the earthly representation of faith, but it is naturally distorted by those that preach it because they are guided by their personal perspective and their interpretation, not necessarily the actual word of the divine. We're human, we are flawed, and so are our interpretations.

So, faith is this nebulous science, belief based, communal, personal, distinct, not truly knowable thing. That was a lot of help. How does it work then, in a game at least? Make it concrete by treating it as a checklist for faith in a game. Does it have mechanics that back up those story elements to make them matter? If you look at many games, faith is really just a question of what color robe you wear when you are thinking of playing a healing bot. But, some go further, and Krendel is one of those.

Let's run through that list briefly.

Science: The universe totally works the way your faith says it does. Only when your belief falters or you run into the beliefs of others might this fail. This is represented through agency. When your use of a power coincides with your faith, then you get the result you want, but if you go against it, then you get penalties or the power fails outright.
Belief: The strength of your conviction and the alignment of your actions with the beliefs of the faith empowers you, augmenting your faith powers or preventing them if you've turned away. Part of this goes hand in hand with faith, but there is another component. If you violate the beliefs of the faith, then you may find yourself cut off, unable to use powers until you atone.
Community: As a priest, you are strongest among the flock. As a follower, you lend your strength to the whole. There is crazy powerful synergy among a group that shares the same faith. A priest may tap into the power pool and power well of her followers to fuel her faith power, and more than a few faith powers are centered around sharing abilities and forging/manipulating/breaking bonds between people. Karma is easier to come by for sharing in communal rites. Plus, it is simply easier to invoke faith powers on those of the same faith.
Personal: The strength of your connection determines what powers you can access and how well. As above, if you ever fall from grace, then you may be cut off until you atone.
Distinct: Each faith defines some powers as favorable, which you can easily acquire. Others are disfavored: they can never be acquired. All others are neutral: they can be acquired with greater effort. Each faith also defines what tenets must be upheld as part of the belief system. This means that although some faiths will have similar powers and/or tenets, no two will actually be the same (if they are, then its one faith, not two, and any differences are in the religions).
Inscrutable: The spirit of a faith's beliefs and tenets is always more important than the letter, the inaccurate representation of those beliefs through words. If there is ever conflict, the Game Master makes the call. Because it is not always clear what the divine whats of us, priests may seek guidance of the divine to help ensure they do not stray from the path.

Of all the power methods, Faith takes the most prep work. At a minimum, you need to work up its style, its beliefs, its tenets, its relationships with other faiths, at least one symbol, a list of 3-5 favored elements (plus universal), and a like number of disfavored elements (not counting universal). You should also give thought to holy days, offerings, religious divisions, history, important figures, consecrated grounds, and more. Yet, all that prep works adds an incredible amount of flavor to the setting.