I think for me it was a gradual slide.
I started out in a Catholic household, where going to mass every Sunday was mandatory unless severely sick. Didn't go to catholic school, but had the Catholic education. I was an altar boy for about six years -- I felt "guilty" about telling the priest it was time for me to quit.
Now it is important to mention I have no real ill will to anyone I personally encountered in the church. I have very positive experiences with all the various priests, bishops, brothers and nuns I have encountered. They were all, to my memory, intelligent, caring and thoughtful people. We had one priest who I thought was interesting--though most of the congregations found his sermons went way above their heads--his weekday job was teaching theology to Jesuits. He spoke of doubt, and he spoke of faith and spoke about that struggle. I think it made people uncomfortable.
That said, somewhere in my mid to late teens I first realized my differences with the political aspects of the church. At the time, for most of my life I saw the church as, basically my foundations of liberal thought. But there was a new Pope (John Paul II). certain aspects of liberalism were no longer part of the church. Not it seemed that the church needed to push its prestige and political clout and engage more into the culture wars. If a parish could afford it, new fancier churches were built. If the parish was poor, the school would close and so would the church.
So I drifted, still Catholic in my cultural identity in many ways, but also more interested in comparison of religion. I became very interested in myths, magic, etc, I would not say that I was dabbling in the occult in terms of any practice, but I found the whole thing interesting.
I would say at this point on my life I was at my least skeptical. Ghosts? Sure, why not? Bigfoot? Maybe. Extraterrestrials? Made sense.
I wasn't a believer in any of these, but had no tools for having real doubt about them.
I think more serious doubt about christianity came about in college. The more I learned about religion the more it looked fishy. I still have belief in Jesus though.
That pretty much got the ka-bosh not long after meeting my wife. Her parents were (modern) Orthodox Jews. She wasn't particularly religious or had any interest, but is very culturally jewish. I learned a lot about Jewish though and learned even more about the "why" regarding their stance on Jesus. It is pretty sound, in terms of the textural history and theology. They reject him as messiah for a host of significant reasons, but usually have zero interest in relating these reasons to christians (why get your ass pogrommed for no reason, right?)
I think about this time I had become just about entirely atheist, but did not use that term to describe myself--I still clung to "agnostic" as I considered "atheist" too confrontational, as I was using a different working definition. than I use today.
Now I usually, if asked by a stranger, will say I am "not religious" but if pressed will call myself an atheist and an agnostic.
I have no faith, and think the ability to know or to prove/disprove the existence of god is futile.
