a word, sound, or custom that a person is unable to pronounce or perform correctly. It is used to identify foreigners or those who do not belong to a particular class or group of people.
It's as close to an objective fact as facts about these things get: religious beliefs are crazy. A quick tour of the world's religions would include gods, angels, ghosts, demonic possessions, talking animals, prophecies, virgin births, afterlives of all sorts, revelation, reincarnation, transubstantiation, and superaquatic perambulation, to name just a few. And that doesn't even include creation myths, which are a particularly rich source of absurdity.
When I realized what was going on here, it kind of blew my mind. The craziness of these beliefs isn't a bug; it's a feature. Religious beliefs are necessarily crazy, not just incidentally crazy — because only crazy beliefs can be used to distinguish members of the community.
A good religious belief allows us to answer the central political question: Are you with us or against us? (Are you on our side? Are you one of us?) This is why questions about belief are especially pronounced when discussing religion across a divide — atheist/theist, Mormon/Catholic, etc. What you believe is a badge of membership in a particular tribe. Which badge you're wearing tells me whose side you're on. 
This is all well and good, but you might think that belief in the absurd would be hard to stabilize. There's a countervailing force pushing against these beliefs: by supposition, they fly in the face of the facts. Wouldn't the crazy beliefs of a group tend to evaporate over time?
Actually, it turns out to be easier than you might think for a group of people to stabilize belief in the absurd. To begin with, the crazy beliefs that we're talking about aren't really falsifiable. They (mostly) don't make concrete predictions, or lead to obviously maladaptive behavior. They tend to be about the past or an unspecified future, or about the ineffable or supernatural (read: nonexistent). Some, like transubstantiation, are (by construction) impossible to verify or refute with evidence. Others, such as the specific nature of the trinity, are so nonsensical as to be practically devoid of meaning. Are the invisible gremlins living in the engine of your car pink invisible gremlins, or orange? It may make a world of difference to believers, but it's hard for a scientist to care one way or the other.
There's also a powerful stabilizing force at play in the kinds of communities that maintain crazy beliefs, namely moralistic punishment. Moralistic punishment refers to the norm of punishing not only cheaters, but also those who fail to punish cheaters. It's a powerful mechanism, and when trained on certain forms of cheating (e.g. not paying taxes), can be a force for tremendous social good. But a community that's good at moralistic punishment can also aim their punishment apparatus at more arbitrary behaviors, such as professions of belief. Once a moral community decides that "wrong belief" (i.e. heresy) is a form of cheating, there will come to bear on those beliefs a tremendous social pressure. A community that treats heresy as cheating becomes capable of believing (and reinforcing belief in) almost anything. 
(Under this view, the craziness acts as a barometer for how strong the moral community is — how tightly the group is able to circle around its sacred center, how much it's able to suppress its members' common sense. The particular craziness of e.g. Mormon beliefs is a signal that the Mormon moral community is exceptionally strong. To maintain such beliefs in the modern era, in face of the world, is quite the feat of solidarity.)
Thus there is a symbiotic relationship between the beliefs and the moral community. The moral community allows the crazy beliefs to come into being (and stabilize), and those beliefs in turn help define and reinforce the boundaries of the moral community.
There's an analogy here with spectator sports. Precisely because there are no substantive reasons to prefer the A's over the Dodgers, your loyalty to a specific team serves as an excellent signal of loyalty to the local community. If there were substantive reasons to prefer one team over another — if the Dodgers were more entertaining, say, or gave out $100 to anyone caught wearing their apparel — then being a fan would reflect only your selfish interests, rather than your loyalties to a particular community.
Of course, fans (or religious adherents) will verbalize all sorts of reasons for their loyalty (or beliefs), but those reasons are largely confabulatory. Statistically speaking, you're overwhelmingly likely to root for your local team, just as you are overwhelmingly likely to absorb the religion you were raised in. You may give reasons later in life for being a Christian or a Muslim or a Buddhist, but in most cases, the only reason with any causal/explanatory power is your upbringing.
I should note, in closing, that there are many religious beliefs which don't function as shibboleths — beliefs which serve object-level, functional purposes. These just aren't the beliefs we usually label as crazy. For example, most religions believe you should treat other members of your tribe (however defined) with respect, that you should put others' welfare before your own. Religions also encode lots of different beliefs about human nature, usually in narratives and laws. Many of these beliefs may be wrong, but many are surely correct and worthy of serious study.
 beliefs as badges. C.f. Robin Hanson's great metaphor on beliefs as clothes.
 moralistic punishment. Much more here.