I suppose I'm an atheist because I had a privileged childhood. Not in the usual sense of that phrase--my family was working class, unintellectual, unartistic, unimaginative--but in an arguably more important way: I had the privilege of growing up without religion.
My family was vaguely Protestant, and if asked by a pollster if they believed in God, both of my parents would likely have answered, "Yes" (which is why I've never trusted those polls that purport to show extraordinarily high levels of religiosity among Americans), but there was no compulsory religion in my childhood home. My parents didn't attend a church (and there were plenty to choose from in Lima, Ohio, from Catholic to Lutheran to Methodist to Mormon to the Kingdom Hall of Jehovah's Witnesses); hence, I was never forced to go to Sunday school; and our living room bookshelf held a set of encyclopedias instead of a traveling salesman's Bible. There was no hostility toward religion in my family; in fact, my parents had a positive attitude toward religion in theory; it was the practice of religion in our Midwestern Bible Belt town that they saw as a hotbed of hypocrisy.
Since I didn't suffer the early implantation of religion in my brain, I was able to adopt a critical attitude toward it as soon as I began to think. At a very young age, I perceived that the consolations of religion were about as credible as a Roadrunner cartoon. I watched my grandparents' coffins disappear into holes in the graveyard ground, and saw no reason to believe that some part of them had magically escaped from their bodies and Houdinied its non-substance into an imaginary realm where my sickly Grandma and bearish Grandpa were transformed into airy angels. Not bloody likely. And no more likely, if much more creepy, was the idea of God as transcendental voyeur, a theological NSA spying with Santa-like ubiquity on our most private doings (including doings done behind bathroom doors). Even less likely was the idea of a loving God sadistic enough to crucify his only child and then spend the rest of his immortality demanding that the creatures he supposedly loves follow a set of absurd rules or be condemned to eternal torture. (I especially like the prohibition against seething a kid in its mother's milk; I'll never do that, I promise.) It is a very strange notion of love with which Christianity warps its believers.
The clincher, though, was that simple and brilliant question every child asks: If God made everything, who made God? Thus does every five year-old refute the First Cause argument. The arbitrary theological closure of the chain of causation seemed deeply 'unfair' to my child's mind, the equivalent of a frustrated adult answering my innocent (if irritating) inquiries with a curt, "Because I said so, that's why." I had much the same reaction to the climax of the book of Job, when God justifies making one of his believers' lives a living hell (in order to win a bet!) by telling the tormented Job, essentially, "Who are you to question me, you pathetic human?" A God who could justify himself no better than my Dad was surely not worthy of worship.
So I was a confirmed skeptic even before my first encounters with the two dead Brits who would turn me toward agnosticism, Charles Darwin and Bertrand Russell. In one important and purely inadvertent way, religious fundamentalists are absolutely correct about Darwin: the concept of evolution by natural selection is an assault upon the religious worldview. In fact, it's an idea that renders religion unnecessary (just as the cosmological theories of contemporary physics do the same thing on a universal scale). We need no anthropomorphized creator because we have been 'created' by an unintelligent process working over millions of years, the same basic process that has 'created' all other life on our planet. When religion fights evolution, religion is fighting for its life, and that fact is the most convincing and eloquent testimony for the power of evolution to dissolve religion like a teaspoon of salt in a glass of water.
Darwin would probably have led me directly to atheism if I had not also, at about the same time (in junior high and high school), discovered the writings of Bertrand Russell. Lord Bertie was my first intellectual hero. His scathing wit and great good sense, wonderfully paraded in such essays as "An Outline of Intellectual Rubbish" and "Why I Am Not A Christian," brought me to agnosticism as the most seemingly reasonable and liberal and tolerant of religious positions. Since the question of God's existence cannot be positively or negatively answered with logical certainty, we should pass over it in Wittgensteinian silence and respect the beliefs and believers of all religions while accepting none.
This agnostic pluralism flowered a few years later (during my college years) into a comparative study of all the world's major religions. I read the Torah, the Bible, the Koran, the Bhagavad-Gita, the Upanishads, the Ramayana, the teachings of the Buddha, scholarly studies of South and East Asian religions, books on Zen, books by Confucius and Mencius and Lao-Tse, the Tibetan and Egyptian books of the dead, Joseph Campbell on mythology, Gershom Scholem on the Kabbalah, scholarly and popular books on the history of Christianity (Elaine Pagels was a favorite); and all of this study resulted in three successive realizations. First, I quickly realized that I now knew more about religion than most believers, very few of whom know even the most basic historical facts about the religions in which they claim to believe. (Ask any Christian to name the approximate dates at which the various books of the Bible were written; I'll give very good odds that he or she won't have a clue.) Second, I was drawn to Buddhism as the only one of the world's religions that accorded value to a consciousness of nothingness, the sense (which I personally experienced in a psychologically decisive way) that an abyss of unmeaning underlies all that we think we know. Third, I came to understand this attraction to Buddhism as a kind of 'atheism by other means,' a bad faith way of embracing unbelief under the umbrella of a vague and trendy spirituality.
My arrival at this last position prepared me for the final and decisive step, my discovery of Jean-Paul Sartre's "Existentialism is a Humanism" and Albert Camus' The Myth of Sisyphus. I have since learned to spot the weaknesses in both these texts, but in my early twenties they were exactly the intellectual fuel I needed to launch myself out of a weak, wishy-washy agnosticism into a tough-minded and pugnacious atheism. These books (and others, by Beauvoir, Beckett, Genet, Faulkner, etc.) also validated my experience of nothingness by positing a world in which we must freely act to create ourselves in the midst of absurdity and meaninglessness. Our existence was a matter of pure contingency, and it was our duty to make from the bare fact of this existence a meaningful life. The world modeled by atheist existentialism was closer to the world of my experience than any of the other worlds modeled by philosophers and theologians. To me, reality appeared more Sartrean/Camusesque than Kantian or Hegelian, Cartesian or Christian, Foucaultian or Platonic. So I am an atheist, finally, because atheism is the idea that best fits the world. It is an idea that liberates and inspires, that gives nothing to imaginary gods and demands freedom for all human beings. It is quite possibly the most revolutionary idea in all of human history. It is an idea for our time.