I was brought up without the benefit of religion (raised in Communist China for 8 years). The closest thing to religion were exposures to stories of deities, spirits, monsters and ghosts from books such as Journey to the West or Liaozhai Zhiyi. After moving to the U.S. with my parents in 1992, we started attending church in the local area (due to insistence of some religious family friends). Hearing the adults say that they believed in an omnipotent god who lives up in the sky, who loves people yet has been known to wipe out almost the entire human population with a flood and then punish the unlucky souls for eternity. It was as if an adult had told me that he/she believes with 100% faith that the Monkey King of Journey to the West absolutely exists and lives up in the sky watching down on me...even I had the good sense to understand that stories are just stories.
I remember a bible study session where I told the teacher that I did not believe in a god. She took my statement very seriously and pulled out to the hallway to ask me more questions - my English comprehension skills were questionable at that time but I could tell from her tone that she was not happy. At the end of class, I decided to tell her that I changed my mind and believed in a god as I was somewhat afraid that she would tell my parents that I was a bad child if she thought I did not believe.
In high school, I attempted to become religious during of a period of loneliness after a sudden relocation. I truly attempted to believe - I read parts of the bible on my knees. After a month of praying, reading and feeling increasingly awful, I gave up. I came to the decision that since I was introduced to the concept of god at age 8, I had probably passed the period of brain plastiticy that allows me to ever "truly" believe.
I have been happily atheist for about 10 years now and have no qualms about telling others when asked.