This is going to be long.
There are a number of reasons I’ve decided I’m an atheist. I have spent a substantial amount of time thinking about this, and reading, and observing, and testing various hypotheses, over the last few years. The more I learn, the more confident I am that my judgement is accurate.
There are many different angles on this, and reasons for thinking the way I do, so for simplicity I’ll just address two of the bigger ones in detail.
First: Every human society has a supernatural explanation for The Way Things Work.
Whether it’s a single all-powerful loving deity, or a pantheon of multi-armed spirits, or a giant winged serpent, or whatever, the one thing all human tribes have in common is some supernatural narrative that purports to explain how the world works and how we’re supposed to behave in it. There isn’t a single culture that lacks this.
These stories vary widely in the details, but they all have three important elements in common. One, they tell us where we came from. Two, they give us rules for interaction and behavior. And three, they tell us what happens after we die.
I find it extremely telling that every human society has its own set of myths and legends covering these exact subjects. There are two possible conclusions: Either there’s One True Explanation, which was given to one small set of believers, and Misleading Falsehoods were planted in the other societies (either by an ineffably mischevous Good Deity or by a malicious Bad Deity); or all human beings share qualities that cause them to manufacture these beliefs. The first possibility cannot be tested; the second, indirectly, can.
I ask the question: Are there aspects of human psychology and behavior that could lead to this sort of mythmaking? And, for myself, the answer comes back, emphatically yes.
First, I note the human drive to answer and explain. Something happens, and we ask, Why? What’s the cause? Starting with circumstances X Y and Z, what is the likely outcome? Obviously, this would be a major survival skill at the dawn of agriculture, if not before. And lately it’s taken us into some incredibly obscure areas, all the way down to the interior of the atom. Do we need this knowledge to feed ourselves and reproduce? Of course not. We simply need to know.
And yet, we all die. What happens then? Nobody knows. It’s been an obsession of sentient humankind for as long as we have history, and it has defied all our attempts at explanation. To borrow a metaphor from a previous post, it’s the UFO we’ve been trying to analyze for as long as we’ve been awake. Except it’s more like a big UFO-shaped hole in the sky, and it defies all our attempts to penetrate the perimeter.
Clearly, we are too driven by curiosity to let “nobody knows” suffice. We have to have some explanation, even in the absence of hard proof.
Look, for example, at how the seasons work: a year-long fluctuation in climate that, if you don’t know you’re on a big rock orbiting a fusion furnace, seems utterly mysterious. Look at how “primitive” cultures, in response to this, created rain gods, or anthropomorphized the sun. Look at how lightning bolts were explained as weapons thrown by the king of the gods. Look at how ancient Chinese astronomers were punished by the Emperor for failing to predict a solar eclipse so forces could be marshaled in advance to make noise and scare away the dragon that was eating the sun.
Clearly, it is well within the bounds of human psychology to invent an explanation, and adhere to it, when actual verifiable knowledge is insufficient to the task. We now know about the Earth’s axial tilt and its impact on local climate variation over the year. We know about the discharge of electricity during storms. We know about the moon blocking the sun from time to time. All of these “primitive” beliefs have died out, having been replaced by concrete, well-tested explanations.
But we still don’t know what happens after death. Is it any wonder that “primitive,” supernatural explanations for it should persist?
The second element of human psychology worth considering is the power of narrative. I find it fascinating that all of the religions and supernatural belief structures I’m aware of capture their wisdom in narrative format, i.e. “once upon a time.” Jesus went here, stood on a hill, and made a speech. Buddha sat on the river, talked to the boatman, and asked him some tricky questions. Muhammad walked across the desert, saw some city, and did some things with some dudes. Apollo descended to earth, saw this chick, and made some nooky. Xenu sat on a volcano, got blowed up, and now a piece of his spirit lives in my pancreas. And we’re supposed to extract our lessons from these parables.
I mean, you’d think God could hire a technical writer to explain the rules more clearly. 
For whatever reason, humans take narrative very, very seriously. Aside from the fact that it forms the basis of every religion I’ve ever heard of, consider how much importance people have attached to worlds as disparate as those of Tolkien, Star Trek, and Arthur Conan Doyle, to name but a few. Vast and complex rationalizations have been invented to account for how, in one story, Watson had been shot in the arm, but in another story, the wound was in his leg (or whatever), and some readers have spent lifetimes figuring out the exact routes, streets and corners, Holmes used in some pursuit or other. Hard-core fans agonize over why Data isn’t supposed to use contractions, except in that one episode where he’s standing between his future and past selves. And just what does it mean to make a Kessel run in twelve parsecs? Hell, some people live their lives by these stories, even though we know they’ve been invented by fallible humans in our own immediately recorded past. Most of the folks who are trying to get “Jedi” added to the Australian census as a valid religion are kidding around, but some, needless to say, aren’t. And where in Eastern Europe were the authorities cracking down on the folks living like hobbits in the hills?
Narrative is power, and it isn’t just in our mass media. Think about how we have to train ourselves to recognize the difference between anecdote and data. Look at how quickly we’re willing to jump to all sorts of broad conclusions based on one powerful but isolated incident, or how a carefully constructed study, with a well-defined cohort and solid longitudinal basis, can be torpedoed in the public mind by a single contradictory tale of woe. For whatever reason, humans are psychologically wired to extrapolate from narrative, and that, to me, puts the whole storytelling basis of religion on a suspect foundation.
So all of that, in my opinion, indicates that humans can be predisposed to invent stories for themselves in order to fill in gaps of understanding, whether or not those stories have any relationship to fact, and to emotionally imbue those stories with an irrational degree of power and importance.
Second: These stories contain no knowledge beyond that of the people who originally believed in them.
You would think that an all-knowing, all-loving deity would be able to give his people all sorts of fascinating and uplifting information. If the Jews really were God’s chosen people, why didn’t he suggest, even obliquely, that they mix together some sulfur, charcoal, and saltpeter, and see what happens when it’s touched with a flaming brand? Is there a single place anywhere in the scriptures of any religion where some transcendant bit of knowledge, something beyond the immediate observational power of the day’s technology and contradictory to the immediate evidence of one’s unaided senses, is transferred from the supernatural power to humankind?
In other words, why didn’t Prometheus come down from Mount Olympus with the germ theory of disease? Or the Copernican model of the solar system? The basic fact that each star in the sky is a really, really distant sun? I’m not even talking about radio transmission or quantum theory or other modern inventions that would have no practical impact on the life of the day. If people knew ten thousand years ago about, say, washing hands before assisting with childbirth, the immediate and verifiable impact on health would be undeniable.
“Okay, Moses, look. These commandments will be useful for shaping your culture, but there’s something more important you need to know, but that you won’t be able to actually verify for, oh, a couple of thousand years at least. This ground you’re standing on? It’s a big ball. No, bigger than that. And the sun in the sky? It’s an even bigger ball of fire and light. And the ball you’re on goes around the sun-ball. Plus, the ball you’re on spins in place. That’s why you only see the sun-ball half the time, because the other half of the time, it’s hidden on the other side of the ball. No, listen, pay attention. The axis on which your ball spins is tilted, so some of the time, you’re pointed more at the sun, and other times, it’s kind of at an angle, which is why you have summer and winter. Isn’t that neat? Now get back down there, the people are being distracted by shiny objects again.”
Where are these revelations? They simply don’t exist. Now, one might rationalize this fact away, saying, “God helps those who help themselves,” or some such pithy maxim. In other words, God knows we don’t appreciate what we don’t create ourselves, or whatever, and makes us figure it out for ourselves. Aside from that contradicting the numerous examples where God does come down and give us stuff (mana from heaven, don’tcha know), what about all those cases where the information contained in the scripture is demonstrably wrong? There are no windows in the sky. Leprosy is not caused by demonic possession. And so on.
The more I look at it, the more it seems self-evident that these religious texts must be the work of their human adherents, because of this limitation of knowledge. The few useful bits of information that are captured here and there, in fact, seem to me to be learned through experience, and handed down as a Rule From God in order to minimize fuss and argument. (For example, the admonition in Leviticus against eating shellfish seems to me to be a trial-and-error realization about not getting sick by eating things that live in proximity to the settlement’s waste outflow.)
I’ve mentioned this in another thread, but the Bible contains no mention whatsoever of tobacco, or the practice of smoking in general, because the ancient Canaanites had never heard of it. There’s tons about alcohol and inebriation, but zip on the broadleaf weed. Likewise, the creation myths of North American natives don’t include the horse, despite its huge importance to their later cultural development. After the horse was introduced, of course, the myths adapted, but the point remains: They couldn’t mythologize what they didn’t know about.
You can take any culture, determine what they did and didn’t yet know about, and verify that such knowledge does and doesn’t appear in their religious beliefs. A tribe in South America? Big snakes in South America? Result, a religion that prominently includes big snakes. Big snakes in Inuit religion? Zippo. The only reasonable conclusion is that the religion is a construct of human imagination.
Oh, sure, there’s lots more. You can look at how our new understanding of DNA has revolutionized our ability to comprehend the world, usually flying in the face of what “religious authorities” have asserted based on their reading of their various scriptures. (The progression of evolution, for example, has been stunningly confirmed by genetic analysis. It’s also remarkable that the biggest variation in human genetics is found in Africa, spreading outward concentrically, exactly as would be the case if the species arose there.)
And you can look at primate behavior, and see how it’s mirrored in ourselves. It makes no sense that a pack of otherwise ordinary teenage boys would beat the crap out of a retarded kid, and enjoy themselves, until you see a bunch of baboons do exactly the same thing on the savannah. Then it’s like a light goes on: Of course.
And so on, and so on, and so on. The bottom line is, when I look at the world, it takes more work to try to impose order and meaning on it, commensurate with the influence of some all-powerful being, than it does to simply accept that all of those supernatural structures have been invented by fallible humans to fill in the gaps of their insecurity and lack of knowledge, and that otherwise we’re a small cog in a vast physical system that has no broad consciousness or intent. In other words, I’d rather accept that the world is the way it seems to be, instead of trying to pretend that there’s something deeper, or to imagine that the way things seem to be is just a mask for something else. It’s just simpler that way.
That’s why I’m an atheist.