I am not a believer of any organized religion, nor am I one of those generically "spiritual" types. I have a few vaguely hippy-dippy pseudo-pagan-animistic impulses (particularly attributing spirits to, and seeking to placate, inanimate objects like my car and my computer), and an unexamined faith in the precept that what one does to others ultimately rebounds upon oneself, though the latter is more a philosophy than any belief in the divine. And unsurprisingly, as a politically liberal, highly-educated (note: educated, not smart!) modern woman, I have a lot of problems with the doctrines of certain denominations and the behavior of many in the name of religion.
Yet I cannot embrace atheism, and ultimately, it's because atheism as a rejection, even hatred, of religion comes into conflict with my principles/beliefs as a historian.
Before grad school, I was pretty anti-religion, and considered almost any expression of religion as an imposition on my right to be religion-free. I'm not sure why I was so hostile, except that I had very little exposure to it. My parents both grew up devout (Anglican on the one side and Roman Catholic on the other), but it was a devotion rooted in time and place, and by the time I came along both my mother and father had left behind the communities and class in which their faith had been fostered. My mother made an effort to teach my sister and me about Jesus' life, but without the parish framework with which she'd grown up, or something to replace it, in her own words, "It just sounded...silly." She told my father she had no objection to our being raised Catholic, but he would have to be responsible for it - the man who never again took communion after his divorce from his first wife. Besides, in my parents' marriage, the immediate, day-to-day tasks of raising the children fell to my mom. So it was nothing, pretty much.
We also heard a LOT about the Holocaust in my elementary/junior high school days, which I think taught me a disdain for religion as much as anything else. Not because the Nazis were portrayed as Christians or representing Christianity - which is good, because I don't think that would be very accurate - but because it was the fact that they cared so much about someone else's religious beliefs that started the whole mess in the first place. (It was okay to be religious - the Jews were cool - but not okay to care about other people's religion.)
What changed in grad school? First, I met some deeply religious people, which opened my eyes to the role that religion could play in someone's life. I remember confessing to a devoutly Lutheran friend of mine that religion just looked like a whole bunch of rules about what you couldn't do, and she just laughed - wasn't shocked, wasn't upset, just thought that was one of the funniest things she'd ever heard.
But more importantly, I began to study the Middle Ages in depth, and ran right up against the fact that whether you think religion is good or bad or neither, it was central to medieval people's lives. And I don't mean that in the simplistic, "they had to think the way the Church wanted them to" caricature that many people hold in their heads in the section labeled "Middle Ages." Rather, Christianity provided a framework for medieval people's understanding of the world around them, a lens through which they saw the universe. This shaped how medieval people thought - in the same way that the modern understanding of gravity shapes how we think today - without telling them what to think. While I've never succumbed to what I've heard people call the professional hazard of medievalism - converting to Catholicism - studying the Middle Ages made me view religion in a more balanced manner.
Because the thing is, if you believe that people of religious belief are, essentially, stupid and irrational, then you have to believe that medieval people were stupid and irrational. But spend even a small amount of time studying medieval people - seriously studying them - and you soon realize this is completely untrue, and in fact, I consider it part of a historian's creed that if you think the people you study were stupid and irrational, you're not a very good historian. That does not mean historians should be cheerleaders for their subjects, or accept or approve of all that they do (hello, I teach about the Crusades!). Nor am I saying that it's invalid to dislike or consider stupid specific historical individuals (my sense is that some biographers come to loathe their subjects by the time they finish their projects; myself, I wouldn't want to spend a lot of quality time with Abelard). But to dismiss an entire culture or society because you don't agree with some of their beliefs is simply not being a good historian.
That's all very well and good, a modern atheist might say, but medieval people didn't know any better. Today we do. I don't blame medieval people for being stupid enough to believe in religion, but I do blame modern people.
But this violates another part of my historian credo - one that is perhaps more important to medievalists and other pre-modernists, but which I would hold to be necessary for all historians. And that is that progress is bunk. People are people are people, wherever and whenever you go. People in the twenty-first century are not smarter or more advanced or more evolved than people in the fourteenth century, just by virtue of the century in which they live. We, like medieval people, interpret the world around us in light of cultural frameworks that shape our beliefs just as strongly as the medieval church shaped the beliefs of medieval people. They just happen to be different frameworks.
This is not to say that those differences aren't important, or that I don't prefer living in the modern world to living in the Middle Ages (from what I can determine about the latter). But for me, the issue of living today vs. then is one of standards of living - we have better hygiene, greater comfort, better medical care (I'd still have a gall bladder if I lived in the fourteenth century). We live lives of what medieval people would view as unimaginable luxury. I'd like some evidence, though, that our modern cultural frameworks have actually reduced human suffering and conflict compared to medieval cultural frameworks.
So, throw out progress, and what you get is not the idea that modern people should be more rational and intelligent than medieval people, but the recognition that humans are humans are humans, and that religion is simply part of human society. All human societies. I don't know of any society that genuinely lacks some form of religious belief (and don't give me the Soviet Union; sending religion underground isn't the same as actually abolishing it). Religion may be "irrational," but that's because humans are irrational. I don't think we can change that, and in many instances, I don't think we need to. I have no problem with objections to specific, oppressive religious practices - no one has the right to hurt other people in the name of their religion. But I see no point in railing about the existence of religion itself. The very existence of irrationality isn't a threat to humankind - it's a condition. Let's study its expressions, let's understand what it provides people.
ETA: From some comments about the meaning of atheism, I need to clarify something: Dr. Virago's comment is correct, and this post is inspired by recent conversations at Pharyngula, especially those about PZ's request for consecrated wafers. I like and generally respect PZ, and, to be frank, didn't want to attract the response of some of his diehard commenters (many of whom I also like and respect, just differ with on this one issue), so I chickened out and spoke in overly general terms without linking. But the atheism I'm referring to here is atheism that is actively hostile to religion, that considers it the height of ignorance and irrationality, and considers the eradication of religion as a noble goal. My apologies to those of you who consider yourself atheists but don't fall into this category - I should have been more clear. (I also probably have more to say about progress but that can go in the comments.)