Most of us were raised in some sort of faith.
Church is a part of the communities of America, and church groups often have outings together, camps, bake sales, festivals--it can be a really inclusive experience, if you fit in.
But churches also have serious problems. Bigotry, indoctrination, taking large quantities of money from its parish--and sometimes those things can send people away.
Here were some of those answers.
Short answer: grew up southern Baptist.
When I started getting to the age where I could think critically about things, I just started noticing some inconsistencies. Specifically, I was told that people who believed in other religions would go to hell because they followed the "wrong" religion. What?? When religion is based a lot on where you are born, I just could not truly believe that people would be sent to hell for being born in the "wrong" country.
Money Money Money
Taking a mythology class in college and realizing all the different religions had similar origin stories of how earth and man were created and evolved, but the details differed based on the environment the people lived in and how far along they were technologically. Anything science couldn't explain was the will of a higher power.
Not saying higher powers COULDN'T exist, just realized people are killing each other because theirs is the 'right God', but the people don't realize that for all intents and purposes, they are all following the similar lessons/stories.
I also believe religion is separate from its organization structure. I think being spiritual and believing in a higher power is okay - it has helped mankind cope with the terrifying unknown. Anybody telling you that God wants you to donate money to them so that their leader can live in a super mansion or own an airplane is a liar. Last I checked, none of that money ever gets trickled up to a deity.
Being told to not question my faith made me lose my faith, if I cannot question what I believe in to better understand it then it wasn't worth believing in it in the first place. Or they are hiding something they don't want me to find out, which makes me even more skeptical of it.
Also reading beyond the selected passages in my bible study classes? Whoa, there is a LOT of messed up stuff in the bible and you would get in trouble! for reading more than just the cherry picked parts they only wanted you to know. Contradictions Galore!
I was in a hard time for me and I started praying, attending catholic activities and doing lots of other things like these. I wasn't feeling better. Years after I gave up doing these things and started taking care of me by myself and now I'm happy. So the faith gradually went away. I'm not fully an atheist tho, I may be an agnostic but I'm not sure.
A Made-Up God
I started to read the Bible.
Then it was gradual. First I did not believe that the god in the Bible was good and created my own personal god. Then I realized that I was making up a god and started to question if everyone else was doing the same. That's when I started questioning, but felt guilty about it and had conversations with god like "You cannot be angry that I am questioning. If you wanted me to believe you shouldn't have left all of this gaps and contradictions." Eventually I became an atheist.
The Business Of Oppression
The Church did it for me. When I started to understand that it was a business. For profit and for power and for influence. When it started to become political. The final straw was when I started to see how much hypocrisy there is, by way of prejudice, and hate.
When Nobody Helps You But You
Was heavily indoctrinated with young earth creationism as a child. Stopped believing in the literal truth of the Bible in college when confronted with extensive evidence to the contrary. Was still very religious, with faith in the moral truth of the Bible. Got engaged to a conservative Christian in medical school, it ended badly. Realized after the fact that Christianity did nothing to prevent the emotional abuse I had suffered by the hands of my mother growing up and my ex as an adult. In fact, faith actively facilitated it in many ways. Hence, not a reliable source of moral truth, too subject to personal interpretation. Put faith on hold. A year later, realized I was happier not going to church. About another year later (just recently), discovered my beliefs align much more cleanly with secular humanism.
The Lies Fell Apart
I spent my early childhood education years in a Christian school. That age is easy to manipulate. The economy tanked when I was entering 5th grade and I had to go to a public school because my parents couldn't afford to send me to a private school. I gradually started to doubt my faith as I was exposed to people from different cultures and religions. I was told by teachers to be weary of people at public schools because they're awful people who will poison my mind. As I got older the lies fell apart. I became an Atheist at 18 and unfortunately I lost a lot of people I thought were my friends.
People Cycling Out
Church politics put me off a lot. I grew up in generally friendly churches full of well-meaning people, but when I was in my teens there was some trouble with the ministers (a husband and wife couple) that were there at the time. They were a bit too evangelical with their sermons compared to the rest of the denomination, and also had some personality clashes with the rest of the church leadership.
After a few years of listening to my parents gossip and grumble, the ministers were sent off to somewhere else and we got a new one. (Not that my parents had any part in that happening, just that that's how I knew about it at all.)
It was hardly the only reason I stopped believing, but it really made me see how these were all just humans, doing their own things, and trying to make their community the way they wanted it.
Everything's Beautiful, Nothing's RealGiphy
I wish I could point to a specific factual source or analytical process or real, concrete and rational justification for my loss of faith like others in this thread, but I can't.
What happened is that I reached my teenage years after being a dutiful believing Catholic my whole life and just...began feeling like it wasn't real. Suddenly I went to court and saw that the emperor had no clothes. I didn't like church, I didn't like all the youth stuff I had to do, and I put my foot down and declared that I would not be following through with confirmation.
The intellectual stuff and the reading and the Sagan and Dawkins and all the other junior atheist bro phase came later. But it started with simply feeling one day that it wasn't real.