Short answer is, I’m an atheist because I haven’t encountered a persuasive reason to believe that a God, or anything else supernatural, exists.
Long answer is, I was raised to believe that there is a God- the God of the Apostle’s Creed:
I believe in God, the Father Almighty,
the Creator of heaven and earth,
and in Jesus Christ, His only Son, our Lord:
Who was conceived of the Holy Spirit,
born of the Virgin Mary,
suffered under Pontius Pilate,
was crucified, died, and was buried.
He descended into hell.
The third day He arose again from the dead.
He ascended into heaven
and sits at the right hand of God the Father Almighty,
whence He shall come to judge the living and the dead.
I believe in the Holy Spirit, the holy catholic church,
the communion of saints,
the forgiveness of sins,
the resurrection of the body,
and life everlasting.
And I fell away from various aspects of this faith one at a time. The first to go was belief in ‘the holy catholic church' and 'the communion of saints.’ Frankly, Christians aren’t better people than anyone else, and their actions show it. There are good and bad Christians just like there are good and bad people of all faith; there is nothing about the church that is holy or even unusual in any way.
The second belief to go was the belief that Jesus is the son of God, and all that follows from that. C. S. Lewis wrote on various occasions versions of the idea that there are two options: either Jesus was who he said he was, i.e. God, or he was a madman; he can’t possibly have been just an above-average, wise man. I was raised in the middle of all kinds of “proof” that Jesus was God and that he really did rise from the dead, and my rejection of this belief didn’t actually deal with the factual veracity of any of these claims. What I realized was that Lewis’ imagination was too limited. There is in fact a third option: Jesus was an ordinary man, manipulated by God because God views human history as a work of art that is more interesting when covered in blood. History makes ever so much more sense if you don’t try to wedge it into a worldview that includes a good and loving God.
Along with the idea that Jesus is God, I abandoned the idea that there is a God who is interested in me personally. There have been times in my life where the smallest intervention would have made the difference between hope and despair; and I don’t mean “small” miracles, I mean the little coincidences that are so often used in churches to support the idea that God loves each of us personally. A smile, a kind word, a hopeful dream, the sort of thing that people often claim God does all the time. The year I was 17, I spent a lot of time praying for some small sign that there was a God who cared, but nothing ever came. When you go to church on a regular basis, you're told all the time that you have to pray and read your Bible consistently because you have to have a relationship with God and relationships take persistent work, but it was like trying to have a relationship with a rock… or an imaginary friend. Eventually I gave up.
I guess what it comes down to is that this is just the way my mind works. I am not capable of faith. I am not able to subscribe to an ideology that I know isn’t supported by any kind of evidence. Things have to make logical sense to me; I think things through and reject my emotional reactions in favor of ideas that I can support with evidence (not just in the religious arena, either: sometime I should write about my hopeless fondness for anarchist political philosophy). I’m not always right; my logical reasoning is sometimes flawed, but I still have to try. Its just the way I am.
And honestly, I think its a good way to be. Life doesn’t consist of the world the way we want it to be, it consists of the world as it is. Emotion is an important part of being human, and intuition and faith and all that are an important part of the way the human mind works, but in order to be a successful person, you have to be able to deal with the world as it is. That means dealing with facts, facing fear and pain, and, when you tell stories about the world to make it seem a more hospitable place, you have to understand what is story and what is real.
I think this also answers, at least partially, the question of what exactly I mean when I say I am an atheist. I believe that the world can be discovered. I believe in reason and science and a way of looking at the world that requires facts before conclusions. There really isn't an atheist orthodoxy that I follow, but a better writer than I put it this way:
An atheist's creed
I believe in time,
matter, and energy,
which make up the whole of the world.
I believe in reason, evidence and the human mind,
the only tools we have;
they are the product of natural forces
in a majestic but impersonal universe,
grander and richer than we can imagine,
a source of endless opportunities for discovery.
I believe in the power of doubt;
I do not seek out reassurances,
but embrace the question,
and strive to challenge my own beliefs.
I accept human mortality.
We have but one life,
brief and full of struggle,
leavened with love and community,
learning and exploration,
beauty and the creation of
new life, new art, and new ideas.
I rejoice in this life that I have,
and in the grandeur of a world that preceded me,
and an earth that will abide without me.