I was born to secular parents. My father is a deconverted Baptist, and my mother a deconverted Jew. While they did expose me to Christianity and Judaism - the latter more than the former, since my father's religious upbringing was extremely dysfunctional - it was more in the interest of acculturation than imposing belief.
When I was thirteen, my parents left me in the care of my father's brother while they traveled on business. Seeing his chance to save my soul, my uncle forced me to watch creationism videos, lectured me on the Bible for five or six hours at a sitting, denied me food when I was hungry, and forced me to attend church services, including a Jews for Jesus sermon where he took me to see the pastor afterward. When I told that pastor that I was withholding judgement until I was older and knew more, he told me that delaying my decision was like knowing that I had cancer and withholding chemotherapy because I didn't want to be cured until I was 18.
When I told my parents what was going on, they cut their trip short, catching the earliest flight that they could buy tickets for. I have never forgotten watching my father lift his own brother up by the shirt collar and throw him out the door onto our front lawn. Needless to say, for many years afterward I was very biased against Christianity in all forms, and still am to a certain extent even now. (Living in a heavily Christian, anti-atheist country really doesn't help.)
When I was 17, I met my first real atheist while working in a summer internship in meteorology. I loved her American Atheist pendant, her belligerence in facing off against woolly-headed theist forum-goers, her zeal for science and reason and secularism. She provided a touchstone for my nebulous adolescent rage, teaching me to channel it into battling the forces of ignorance. That summer was the happiest of my high school life, because of the fellowship I found.
Formative experiences aside, I never had a particularly strong interest in or desire for theistic belief. I contend that, even if my experiences had not been so polarized, I would have ended up the same - tolerant and open-minded, but an unbeliever nonetheless.