Hey, saw a bunch of these and decided to add my two cents.
I was raised Catholic and kind of sort of believed in the way a person who doesn't know any better does. I made sense to my 5 year old brain and everyone else believed it. I certainly hadn't been exposed to any different points of view. In fact, when I had a Baptist friend tell me I was going to hell for being Catholic I was more confused than upset. "Wait, you're not Catholic?" That being said there was a bunch of stuff I just never accepted. No one was ever able to convince me that the little cracker we ate *actually* became the body of christ. I flat out refused and said so. But my folks just laughed it off as being an ignorant child. Skip ahead a few years...
I've always had a deep abiding love for science, and biology in particular. I love life! I dove head long into education and reading. I found out about dinosaurs and earthquakes and evolution! I started asking questions and causing problems. Why were there no dinosaurs in the bible? Why does the story of genesis conflict with evolution? At the time I had no idea what I was trying to say but during one argument with my mother I declared that I was a scientist. I know now I was trying to explain a naturalistic world view and methodology, but at 10 I hadn't gotten past, "but this makes sense!". You learn quickly to just shut up and take it when you call the bible 'just a book' and get threatened with the removal of all educational material. You couldn't have thought up a better punishment than taking all my books. :(
Around 13 or 14, high school age at least, I decided I didn't believe in god. I didn't know the world atheist but I knew I wasn't Catholic and well, what else was there? I just couldn't tolerate any worldview where a person is born so awful even god can't stand you but deigns to save you anyway. I knew I was awesome dammit! Any Catholics know what I'm talking about. The first part of mass (in a creepy, brainwashed way) is for the whole congregation to stand and confess how horrible we are for being sinners. I believed that all babies were tabula rasa. I guess I was a little bit humanist all along.
Well of course, I didn't tell anybody about my new found non belief. After years of arguing with my parents I learned you pretty much don't tell authority figures anything of importance. (I still feel this way about my parents)
However, here's where it gets funny! I met a wonderful group of wiccan/neo-pagans my senior year of high school. It seemed to answer questions I didn't know I had while accepting nature in all it's glory. I found the scientists religion! I was hooked, and I went from being a non-believer to a gnostic theist in the divine goddess. I was more into this religion than I ever was christianity. It was fun to keep track of the changing seasons and believe in the connectedness of all life everywhere. We learned to see auras, which I took as proof of god. (still needed proof at the time, just didn't understand the different qualities of proofs)
I'll skip the next 10 years of being a die-hard pagan. (never liked the term wiccan) But my close friends called my Buddhist for some strange reason. But I was happy and things seemed to make sense, until I stumbled across an episode of the Atheist Experience on youtube. It was fine at first. "yeah, make fun of those stupid christians". Until, I realized that all the reasons I believed where thoroughly knocked down too. Within the space of a few weeks I went from being a gnostic theist to an agnostic atheist. I still have a soft spot for nature religions/Jains/Buddists but no longer feel there needs to be a spiritual side to fill in the gaps of our knowledge.
TL;DR? Went from agnostic atheist => gnostic theist => agnostic atheist