The Creation book raised serious doubts. Then looking into the flood myth and the epic of Gilgamesh. After that I was agnostic. Then I found the James Randi Education forum and argued religion on there, then I read books like Combating Cult Mind Control by Steve Hassan and God is Not Great by Christopher Hitchens and finally ended up at Atheism.
One of the main factors was you're not born knowing that God exists, someone tells you that he exists. How do they know? They read it in a book. How did the writers of the book know? They say that God told them. How do we know God told them? The writers say so. This means that to believe in God you actually have to have faith in men. The person that told you that this book's God is the right one, the person who tells you that their interpretation of the book is the only correct interpretation, the people that wrote the book (and the people who translated the book, and the people who printed the book), you have to have faith in all of them first before you can believe in God.