I'm a new a born again atheist :P - as of well, a couple weeks ago. Raised Christian my whole life (25 years...woot!).
Me, I grew up learning about Jesus and God and so forth, and then as I started going to church more and more, I would realize that I just didn't agree with things I was being told to say I believed (the Nicene Creed, etc). So I would just not say that part of the service. Then I found myself more and more uncomfortable by things said in the service - things that I categorically did not believe to be literally true - to the point where it just felt so false to be attending.
So I didn't attend any church for a good long while. I considered myself agnostic, mainly because I find being agnostic to be "safer" both socially and emotionally. Being an out and out atheist is so difficult in the US, that it took me a long time to "go there", as it were.
My dh helped. he is an atheist and the more we talked, the more I realized that really, truly, I am too.
it was a gradual process and only in the last few years (around age 35 or so), did I feel comfortable stating and knowing that I am an atheist.