OMG! Wait…no. Yours.
By now most of us have seen the results of Pew's Religious Knowledge Quiz, which finds that atheists and agnostics tend to be more knowledgeable about religion than the traditionally religious. I have seen and heard some speculation as to why this might be; from believers, the explanation seems to be some variation of "They didn't ask the questions to which I know the answers" (though the correct answer is "I don't know much about religions other than my own." There's a certain conceit in reading the phrase "Religious Knowledge" as synonymous with "Bible Knowledge". ) I might be tempted to say that atheists and agnostics are just smarter, but this is clearly not the case; to paraphrase Michael Shermer, smart people are often just better at rationalizing their non-smart beliefs (though I think it can be safely assumed that atheists and agnostics tend to be less credulous). My own analysis is simply this: serious religious study tends to lead to atheism or agnosticism.
Of course, I can only speak from my own experience, though it seems to have been mirrored by many of my peers. When I was growing up, what passed for "Bible study" was often little more than an exercise in revisionist history and self-serving rationalization. Historical and cultural context was not only largely ignored, it was often a point of pride to do so; the Bible "means what it says". Even in more liberal congregations, there was always a tendency to read scripture in light of current theological thinking, rather than in the context of the particular environment in which it was written. Some of that context can be regained by simply reading the Bible linearly, from front to back, rather than by "verse-hopping" in order to prove a certain point. Such a reading also highlights certain contradictions about the very nature of God: God is eternal and unchanging, yet the anthropomorphic, tribal war god of the Old Testament bears little resemblance to the formless, transcendent, loving God of all nations of the New (for that matter, the very existence of an "Old" and "New" testament puts lie to the notion). God is both completely good and completely moral, yet his behavior is sometimes decidedly immoral. The Jews were always strict monotheists, yet the texts indicate they were, for much of their history, monolatrists (never mind the vestiges of polytheism). God promises eternal punishment or reward, while early Jews seem to have no concept of the afterlife. Christianity is exclusive, but prior to its advent, Judaism is trending toward universalism. What's more, the change seems to be politically motivated; post-exilic Jews want to make nice with the rest of the Persian Empire by equating their gods with Yahweh, while Jewish nationalism seems to have made a resurgence by Jesus' day. How can an obviously evolving faith claim a monopoly on truth? How many times can you see yesterday's God supplanted by today's before you begin to doubt today's God as well?
But it's in their knowledge of non-Judeochristian religions that non-believers claim their edge in this survey. As knowledge of other gods and myths and cultures expands, we finally come to the ultimate question: why should I whole-heartedly accept this particular collection of story and myth, while denying this other, very similar but mutually exclusive collection of myth? We might come to the conclusion that the distinction is arbitrary; there is no real reason to prefer one over the other. Yet, they can't both be true. They can, however, both be false.
But that's my story (never mind the philosophical and scientific objections). I can also imagine further explanations of the data. A non-believer may simply have a legitimate desire to understand this "thing" that is obviously so important to so many people (I admit to a degree of this, and to a curiosity about the nature of belief in general). I will also concede that there may be a measure of "eye-poking" going on, of non-believers looking for ammunition to use against believers, though I think this is rare (most non-believers I know are not at all evangelical about it). It should also be mentioned that many self-described atheists are rejecting a particular conception of God, usually the one they were brought up with, rather than the notion of God entirely; a person dissatisfied with his own religion might be expected to do a certain amount of research. That's precisely why many religions are so insular, and hierarchical, and authoritarian; because, the more you know, the less likely you are to believe.