Do you believe that rain falls down? Now, do you have faith that rain falls down?
If you see someone standing in the street asserting that rain falls down and not up, you might wonder why someone would feel the need to state the obviously true. But if you see the same person standing in a thunderstorm, declaring with conviction that rain falls up as they feel it coming down on them, that's faith.
A statement of faith need not be false, but it must be implausible, and held passionately. The parents of a child who disappeared 20 years ago might cling tight to the belief that their offspring will return some day, and we recognise that it's not impossible that it will indeed happen, while acknowledging it's not at all likely. But if we saw the same parents casually holding the same belief but attaching no importance to it - that would seem profoundly strange.
So, faith must be passionate, and its objects unlikely. We don't even have a special word for beliefs which are mundane and held without intense emotion.
Indeed, the more implausible a belief, the more faith is required to believe it. This is why the most fanatical believers are also the ones with the wackiest beliefs.
But to turn this around, the people with the most faith to give are the ones who seek out the wackiest beliefs to expend their faith on.
How do you contain a fanatic? Give them something trivial or irrelevant to be fanatical about. The Turin Shroud is an obvious forgery, but even if it were genuine, its existence wouldn't support one christology over another. In spite of this, many fanatics come up with labyribthine sophisms trying to prove it genuine, as if that would somehow prove their theology.
If these people's efforts were directed toward catholicism itself, the church would splinter and implode. Churches direct potential heretics toward arcane irrelevancies for the same reason wider cultures direct potential troublemakers to sporting trivia.
But what makes one belief plausible and another not?
If I told you there was a teapot orbiting Neptune, and you had no way of investigating whether or not it were true, you could assign equal probability to truth or falsity, or refuse to assign any probability until some evidence turns up.
But actually you'd assume by default there is no such teapot, being ready to change your mind if evidence emerged that it were there.
If I told you there is a man in Brazil who's married 13 times, each time to a different woman named Maria, then it's rather unlikely - but probably not as unlikely as the neptunian teapot.
But if I told you my parents own a dog, you know that dog ownership is common, and that people seldom have reason to lie about such things, so you'd probably believe me.
So although there may be clear rules for the trained mathematician, in ordinary life we use a dense and inconsistent set of vague notions of plausibility to decide. This plausibility structure has aporias and outright contradictions, but for the most part is self-supporting - so we can generally justify our intuitions with other intuitions, which then justify the first set.
Different domains of life have different plausibility structures. In school, what the teacher says is considered probably true, because the teacher says it. Outside of school, teachers know nothing. In church, the priest is a wise guide. But the priest who visits your home is the happiness police, and must be lied to.
So, within religion and its rituals, the absurd and incoherent beliefs of the church become plausible. Miracles as explanations for natural events become commonplace. Statements which no one understands become obviously true - though still not understood.
So there is at the heart of religious faith a tension. On the one hand, articles of faith need to be wildly implausible to be articles of faith. On the other, they're rendered mundane by societal conditions which make them not just plausible but obvious.
Thus when engaged in acts of worship, believers do not have faith. And it's only when not expressing their faith that they have it.