You need a few good stories - though they don't need to explain or account for everything in the universe.

Basically, the first step to creating a believable religion is to create a series of connected stories with a central theme that people would find compelling - EG, freedom from slavery, living in peace and harmony, or showing courage or faith in the face of adversity, or why you shouldn't sass the gods. (It doesn't hurt to have a little of all of these show up in various stories, either.)

The more detail you add, the more believable it will be to the audience, whether they intend to believe it or not. Cosmology (how the world got here, how it came to be the way it is, where it's going, how it's going to end, and all that fun stuff) is an easy way to add a large amount of detail with minimal effort.

Remember, while detail is good, a religion's cosmology doesn't need to account for or explain everything - it just needs enough to support its own stories and to inspire faith in the unseen. (Most religions get along perfectly well without explaining black holes, nuclear fission, or baking soda, after all.)

So what makes for a good story? Let's take Zechariah Sitchen's "translations" of ancient Sumerian texts for example. They may not have been accurate, but they were certainly compelling: his "translations" tell a tale of early proto-humans that were engineered to become the slaves of "gods" that were really aliens, and who eventually left humanity behind and left on their brown dwarf Nibiru, but who would be returning and bringing with them massive cataclysm. It's not a particularly happy story, but it sticks with people, as evidenced by the fact that it's helped bolster widespread belief that Planet X/Nibiru would swing by Earth and cause cataclysm in the near future for decades now. (It's been supposed to arrive as early as 2003, but rather than give up on believing in Nibiru, believers and proponents just push the date back.)

The story of Atlantis is another good example. It was never even intended to be taken as fact (it's explicitly described as "fiction" in Timaeus), but the story of a glorious and proud nation that not only fell but disappeared entirely due to its own arrogance was compelling enough that it was eventually worked into New Age belief, and to this day people who don't subscribe to New Age beliefs continue to look for Atlantis, or a city that may have inspired the story.

One story that's pretty popular on the Internet is the Thrymskvida, better known as "that time when Loki dressed Thor in a wedding gown, dressed up as the bridesmaid, and pass Thor off as Freya so Thor could get his hammer back." This story sticks with people for the simple fact that it's funny - Thor, the manliest of the manly, has to dress in drag while Loki has to make sure he doesn't do something Thor-ish and blow the whole operation.

Also, for an extra layer of realism, don't be afraid to come up with some contradictory stories as well, or stories that tell or explain more or less the same thing in different ways. Legends inevitably change and diversify over time for a variety of reasons. If a single group splits apart they'll each end up with their own variations of the story eventually. Stories introduced to a new audience may end up altered slightly to fit the new audience's understanding of the world better. Furthermore, two close, yet somewhat separated groups may come up with two entirely different stories to explain the same thing on their own.

Things that its original adherents encounter or place value upon are more likely to have significance or explanations than things that are unheard of or worthless.

When it comes to real religions, you'll invariably find that their symbolisms and beliefs are shaped and influenced by the regions they developed in. Things that are highly valued are more likely to be seen as sacred, whereas things that present danger or are associated with danger or destruction are more likely to become symbols of evil or ruin.

In Norse mythology, Ragnarok is supposed to be preceded by three years of solid winter - and in the northern regions where the Norse belief system came into being, cold temperatures are a very real and present threat. The Greeks were pretty big on engineering, so it should be no surprise that their mythology incorporates robots. They were also pretty big on arts and entertainment, hence the muses and the critic god Momos.

For the ancient Egyptians, the Nile was a pretty big deal in their religious belief - which makes sense given the Nile was what enabled them to live in such a hot, dry region in the first place. On the other hand, Egyptian spirituality doesn't have much to say about ice.

So when you're thinking out your fictional religions or belief systems and the stories and symbols attached to them, think about what the people who created them would have been familiar with, and work from there.

It should appeal to peoples' sense of goodness in some way.

Generally speaking, most people want to be good. Now, while peoples' personal definitions of what 'good' means can vary, the vast majority of people would agree that the world would be a better place if we all stopped killing and bickering with each other and started focusing on actually fixing the world's problems. (Unfortunately, the sad fact remains that we will never agree on exactly what is or isn't a problem, let alone on which problems should be given priority. Bickering and fighting are inevitable no matter what happens.)

With few exceptions, people believe that those who need help should be helped and that we shouldn't be wantonly cruel - and this is reflected in the tenets and philosophies of pretty much every religion out there, even some that people would consider to be... ah, unsavory. Basically, this stuff is hardwired into our coding. Any religion that doesn't have good things to say about peace and kindness is going to be in the minority - and then, it's going to have an extremely short shelf-life.

You don't see people lining up in droves to join religions that outright tell them to go and cheat lonely old ladies out of their money. You don't see people jumping to join religions that require followers to kidnap and murder random babies.

Of course, there are plenty of religions that encourage or allow their followers to do absolutely awful things - but the thing is, it won't be framed that way. The lonely old lady might be framed as a supporter of an oppressive system (read: one that doesn't accommodate the religion's whims at the drop of a hat), and those who would cheat her out of her money as Robin Hood-like heroes.

Basically, it should give adherents/believers a reason to feel like the good guys at least some of the time.

Adherents should have differences of opinion.

Religions are comprised of individuals, and this means that even if they are cohesive and cooperative on the whole, there will always be some differences of opinion. Let's say for the sake of argument that the fictitious religion has the following mandate:

On Tuesdays, you will eat fruit and meditate upon the sacred writings.

Jane might believe that this is something you have to do all day, whereas Bob thinks that a few hours will suffice. Carol might think that fruit leather counts, but Lauren thinks it has to be fresh fruit all the way. Patrick might think this means nothing but fruit, but Ted might think it's all right to have anything else you want in there so long as most of your meal is fruit. And then there's Lucy, who's of the view that you just need fruit in there somewhere... (and cucumber is technically a fruit, so...!)

Depending on just how important the practice of eating fruit and meditating on the holy writings on Tuesdays is considered, this could eventually lead to rifts and splinter groups forming. In fact, assuming a group doesn't just die out altogether, rifts and splits are inevitable. If your fictional religion's been around for awhile, consider developing a few different subgroups, each with variations in practice and creeds.

Religions and religious beliefs are remarkably resilient, and people don't necessarily abandon them just because part of it is proven to be false or questionable.

In Fictionland, all a protagonist has to do to get people to stop following a false religion is to dig up a little dirt on its origins, find a few contradictions, or show that some belief or other held by its adherents is patently false. In reality, belief does not collapse so easily. Today, many religious groups are quite happy and functional believing that certain elements of their stories were meant to convey spiritual messages via allegory, rather than literal accounts of history.

Believers in Nibiru/Planet X by and large are not discouraged by the mysterious planet's failure to arrive, but instead just wait for it to arrive on a later date ad infinitum. Similarly, when nothing happened on December 21st, 2012, proponents dealt with this in a number of ways - some "recalculated" and re-set the date a few weeks ahead. Some claimed that the prediction really did come through, though curiously the "fulfillment" was imperceptible from everyday socio-political problems. Some claimed that God delayed it for divine reasons.

If you're creating a polytheistic system, don't make the deities one-dimensional archetypes.

Overall, people have a wide variety of ways to retain their current beliefs against apparently incontrovertible evidence. Dismissing the source of the information as deluded or deliberately trying to destroy the group with lies and misinformation isn't uncommon, the latter particularly among the more paranoid types. Some people will just compartmentalize contradictory evidence away and forget all about it. Other times (such as in the case of those who interpret their religions allegorically) people find way to harmonize their religious beliefs with current knowledge, and the religion thereby adapts and survives.

Fictional deities are often portrayed as one-dimensional in their areas of expertise or influence - eg, you have a "god of war," a "goddess of love," a "god of thunder," and a "goddess of wisdom." In real pantheons, deities are not pigeonholed so neatly. First, deities typically have more than one area of influence/expertise. Secondly, overlap can and does happen - frequently. Let's take Thor, for example. Many folks see him as a god of thunder. In reality, he was quite a bit more than that - he was also a protector of mankind and a fertility god. But he wasn't the only god who dealt with fertility in the Norse pantheon - Freyr and Freya also dealt in that department. In this same vein, Loki wasn't "the" god of cunning and trickery - Odin was also into that, and what's more he was into poetry (though Bragi tended to specialize in poetry). And speaking of Loki, he wasn't just a god of cunning and trickery - he was also a hearth god.

Basically, give your deities the same complexity in skills and personalities as you find in real people, and you'll be good to go.

Don't be afraid to step out of dualism.

In a nutshell, you know that whole "conflict between good and evil, cosmic battle between light and darkness" thing? In the real world, this actually doesn't exist for a lot of religions. Sure, it's a big thing in some of the most popular and well-known religions out there, but it's nowhere near universal. While it's not necessarily bad to use in and of itself (some religions are dualistic, after all), a complex and well-built world would do well to incorporate a few religions/belief systems where duality isn't a very big theme at all - or where it just doesn't exist period.

Let's take Greek belief, for example - these days, writers often present it so that the Olympians are good and the Titans were evil. In actual Greek belief, the Olympians may have overthrown the Titans, but the Titans (lead by Cronus) ruled over Earth during its Golden Age, a time without strife and pain. Of course, Cronus did eat his children to prevent a prophecy that one of his children would overthrow him from taking place, and that's not so nice. But then again, Zeus, who overthrew him, would later go on to imprison and torture the Titan Prometheus for making sure humanity had fire and good food. (Zeus did eventually allow the release of both Prometheus and Cronus, fortunately.) There is no light/darkness dualism here, no struggle between good and evil. It's family drama and political struggle.

Similarly, some recent interpretations of Norse mythology cast the Aesir into the role of the good guys and the jotnar into the role of the bad guys. In reality, it wasn't nearly so simple. The Aesir weren't portrayed as wholly good, but as complex and fallible as anyone else. The main difference between the Aesir and the jotnar was that the Aesir were pro-human while the jotnar generally didn't care one way or the other (and given that they are frequently associated with the forces of nature, this would make sense - nature isn't good or evil, it just is). As far as causing trouble for the other side goes, the jotnar and Aesir were pretty well tied. That said, the Aesir frequently married jotnar women or had a jotnar mistress (or several). And then to add to the complexity, the Aesir weren't the only gods around - there were also the Vanir, who ended up in a war with the Aesir, which resulted in a few of their number (notably Freya and Freyr) joining the Aesir. Again, family drama and political struggle.

Don't be afraid to step out of every other box, either.

In the real world, religions are far more varied than most people give them credit for - so when they all follow the same conventions in Fictionland, it's a little strange.

Religions and belief systems can exist without any gods at all. There are no gods in Buddhism, for example - the whole point is to release yourself from earthly attachments so that you have no reason to reincarnate and thereby return to the suffering of this mortal coil.

Gods can also exist without religion - someone can try to interact with a deity outside of religious structure.

Everyone knows gods are immortal... right? WRONG. In many pantheons (eg, Norse, Yoruban, Egyptian) gods can indeed die. (Though this doesn't necessarily prevent them from being active players in the cosmic drama!)

In Star Trek, it was eventually revealed that in Klingon mythology, the Klingons actually killed their own gods because they were too much of a pain. The good Klingon afterlife is said to be looked over by Kah'less, a legendary Klingon hero. Mortals killing gods sound ridiculous? In the Gesta Danorum, the god Balder was killed by the mortal Hother. A non-mainstream myth, but a myth that exists nonetheless.