I was raised to think Christian orthodoxy. I don't remember having any doubts until a social studies class introduced me to a demographic of the world by religion.
It dawned on me that, had I been born elsewhere, I would have been born into a different religion. Therefore, the result was strictly random, nothing more. There was no plan or design; I was not chosen. In fact, how patently unfair it would be for a God to chose dumb little me over the entire race of Indians, Africans or Chinese. God would have gone around the globe picking out all the saints without regard to race or location, and given them the True Religion allegedly given to me. Who was I to say I was better than most of the population?
So that did it. By the time I got up the next morning, I realized I was an atheist, and began breaking the news as gently as I could to horrified family and friends.
I never once had even the slightest reservation or doubt. It's just a matter of simple logic, and no teaching has ever since even come close to giving me pause. If anything, I have developed a deep aversion to some aspects of religiosity and a strong affinity to all philosophies that respect "logic and proportion" (as it were).
If every human being could be compared to a single cell in a "body politick", so to speak, then the cells that are rejecting logic are rejecting a principal beneficial to the body at large. This would appear to model disease.
I believe the world will someday outgrow this condition. I believe we are just like any other society in history that thought it was living some golden age, on the cusp of an apocalypse, etc. I believe this is just an ephemeral state over the long run, that future generations will marvel at our primitive and sluggish rebellion against evidence and reason.
I don't mean to encroach on anyone's sensitivity to intolerance, but at some point you want to wake up the guy in the seat next to you, the guy with dragon breath who's drooling on your shoulder, hollering incoherently and flailing his arms. You just want to wake him up and tell him, "Hey man, are you OK? 'Cause you just messed up my clean shirt." If that's what atheists appear to be doing to theists, then I would ask: but isn't that reasonable?