What percentage of atheists do you think were churchgoers until reaching some age?
I was a regular churchgoer as a child and through my high school years. And not a passive one. In high school, I became heavily involved in the musical aspect of the church I attended, a traditional Presbyterian church in a small town in California. My parents, who were not at all religions, occasionally expressed concern with the amount of time I spent there. Apparently, though, I took some comfort there and social connections were part of that.
However, I was a young person with an unending supply of questions that had poor or no answers. As a regular around the church building, I had many opportunities to discuss those questions with the pastor of that church...well, until he was fired by the board of deacons. It seems that the pastor's wife had an affair with one of those worthy deacons and the easiest solution was, apparently, to send the pastor and his wife elsewhere.
As I went through my teens, the religious account of how things were and how things happened made less and less sense to me, and I began to question whether this religion stuff was worth bothering with. Being an avid reader and explorer of points of view, I began reading rather extensively about religions of all kinds, including the reasons there were so many different flavors of Christianity.
In the end, none of it really made any sense at all and seemed to fly in the face of actual evidence, so I stopped bothering with it altogether, although my interest in the interface between religion and society never disappeared, and still hasn't.
In the face of a pantheon of deities and beliefs about deities and other such supernatural stuff taken seriously by various people in various places and times, I became an atheist. I could no longer believe in any of that. It wasn't really a choice. I simply could not believe that such entities and tales about those entities could possibly be true.
Other human behavior, such as hypocrisy, untruthfulness and intolerance among the religious people I encountered did nothing to encourage my continued attendance at church or its functions. I simply walked away from all of it, while continuing to maintain an interest due to the heavy influence religion has on society. That interest remains, but the impossibility of belief in such things as deities makes it only an academic exercise for me.
That's my story. I'll be glad to read others' stories, if people are inclined to post them.
= new reply since forum marked as read
Atheist adjacent agnostic here – just to be clear for any future Pence-led inquisition.
I was 6. I got kicked out of Sunday School for asking too many questions about Noah's Ark. I asked: if the children of the animals on the Ark had to marry their own sisters and brothers, wasn't that just wrong? The teacher grabbed me by the armpit, led me outside, and sat me on a stump in front of the church to wait for my mom to come. I remember seeing all the other kids against the windows looking like they wished they could be on the stump with me.
I guess I'm still on that stump. As stumps go, it's very nice. 
... I set out, in my late teens, on an earnest search for "truth". I was a Lutheran, Mormon, Unitarian, Theosophist, Wiccan, Mahayana Buddhist, Theravada Buddhist, and Taoist, not necessarily in that order, and not necessarily in a linear fashion. Then I noticed that the things that I found silly in one religion, were just as silly when viewed through the lens of a different religion.
Then I stopped searching and asked myself what, precisely, I was looking for.
It wasn't fulfillment, because I felt very fulfilled through my family, my career, and my hobbies and activities.
It wasn't serenity, comfort, or an antidote for fear, because I found those within myself after I learned to meditate.
What it came down to in the end was a fear of death, and what happens after death.
So finally, I realized that every minute I spend worrying about death is a minute I don't spend actually living, at which point I said to myself, to hell with it. I'll spend my time and energy on living, and if there's no heaven waiting for me, then I damn well better do everything I can to make my own heaven right here, right now, on Earth.
(And, for what it's worth, I still entertain the notion that there might be such a thing as reincarnation, in which case, I hope I come back as an atheist to begin with and save myself a lot of wasted time in soul searching. 
)
and we are all game-players in that simulation, then we play the game voluntarily, and can enter the game in any form we want to.
(For what it's worth, my earliest childhood memory is laying in the crib looking at my hands and thinking "How strange it is to be small again."
after going to Seminary 