[Copy-pasted with very slight modification from several other places on the internet. My point in posting this is not to disparage believers, but to share my personal story and to help people understand the thought process of at least one person who has de-converted. I attempt to show a respect for the Bible and Christianity.]
My whole life I have continually talked to God and asked him many things. I heard from many Christians that God does listen to prayers and that he responds to us in our hearts. But I never felt anything, ever. Not even many years before I even began to doubt my faith.
My parents and pastor told me I needed to be convicted of my sin. But I believed in my sin, that wasn't the issue. When I asked them what my sin was, they told me that only God knows. I asked them how I could find out what my sin was, if I was unable to ask God to convict me. They could not answer.
A Sunday school teacher told me I probably didn't have enough faith, so I asked him how I could get more. He told me to ask God for more faith. I said, "How can I ask God for more faith if I need more faith for him to respond?" He could not answer me. To date I have not found answers to these questions.
Why don't I believe? There's a logical and emotional side to it. First, the logical side. When I was taking a dual-credit college class in high school on world history, the teacher was speaking of the rise of Islam. He said, "The best way to learn about the Koran is to read it." Agreeing with that sentiment, I read it through. When I was done I thought, "That was in a lot of ways similar to the Bible." Then I thought, "Islam is the second-largest world religion. Many many people believe this to be the word of the one true God. But I have never even considered Islam. Why not? Why am I a Christian and not a Muslim?"
Mark Twain's words rang true, "The easy confidence with which I know another man's religion is folly teaches me to suspect that my own is also." I realized I only believed because my parents had taught me to believe. That wasn't good enough for me. If I had Muslim parents, I'd be a Muslim. I needed a real reason to believe.
I read biblical apologetics books and counter-apologetics, and found the apologist case to be severely lacking. I saw through most of the holes before even reading the rebuttals. At this point in time, I was also reading other holy books (the Hindu books, the Dharma from Buddhism, and the analects of Confucius). I began to see the Tanakh not as a Christian "Old Testament" but as the Jewish scriptures. Reading these books opened my mind to other religions, and I saw that Christianity's morality was not unique. Much of it was taught elsewhere.
I read Thomas Paine's Age of Reason (freely available on the internet) and was convicted of the Bible's lack of integrity. Paine made me laugh and cry as I found my beliefs deconstructed. I became a deist, a believer in a creator God unassociated with religion. I realized that the default position should be non-belief without a reason to believe, and I had only one reason left to believe in God: creation. How could all of this be created without a God?
I knew I had been inculcated in anti-evolution sentiment. If I had abandoned Christianity, I could also abandon creationism, right? Well, first I had to read the case of evolution. I read Darwin's Origin, now my favorite book of all time. I read The Greatest Show on Earth by Richard Dawkins, Why Evolution is True by Jerry Coyne, Evolution by Donald Prothero, Your Inner Fish and The Universe Within by Neil Shubin, a couple of books by Stephen Jay Gould, and finally the more complex book The Red Queen by Michael Ridley. After reading these books, the mountain of evidence convinced me completely. No longer having any reason to believe in God, I became an atheist.
Now the emotional side. Like I said, I talked to God for a long time. Over my deconversion process (thinking about other religions, reading Age of Reason and considering evolution's effect on theism), I actually talked to God even more, even more frantically. It was like when a lifelong friend is moving away, and you try to spend as much time with them as possible. But there was nothing, still. There never has been. I came to the crushing conclusion that I had been dreadfully wrong. I had been speaking to an imaginary friend since I'd learned how to talk. I said goodbye to God.
And then, when I became an atheist, I was suddenly liberated. I realized that the Bible did not have to dictate my morality. I could freely question the Bible's integrity. I could freely question creationism. Actually, I could freely question anything. Atheism has liberated my mind, and I would never go back.