An accumulation of things piling up over time: That's what stopped me from believing in God.
When I was a Christian, personal experience was why I believed. I believed I could talk with him. Yeah, I was filled with that holy ghost shit, and I'd swear my ass to you I saw miracles too: Prophetic words that seemed to come true, an otherworldly euphoria brought on a bevy of cosmically-ancient hymns, and the unexplainable healing of my fellow persons. Had to be Jesus, right?
The healing was the first thing I began to doubt. "Placebo Effect" was the term I heard on TV. I listened, and was hit in the dick with it. It worked via the power of belief, the exact same way God's healing seemed to. Every healing I witnessed was more easily explained this effect, as well as any faith healing outside my religion.
Why could God instantly cure my headache, but seemed to have trouble unjamming my car door?
My second tangled kite-string was the accounts from people of other faiths. If their personal experience told them their religion was true, and I didn't believe them, then could I honestly say my experiences proved anything about the truth of my own religion? Well hecky, that'd make me a hypocrite.
Then like some kind of pious drug dealer, a cousin of mine opened his trenchcoat and said, "Hey, want to try some Ravi Zacharias?"
Christian Apologetics were my drug for a while.
Their solo-talks were one thing, but their debates revealed their true nature. From apologist to apologist, I snapped proverbial pencil after pencil in frustration. It's not that the skeptical atheist side had more convincing arguments, it's that time and time again--when backed into a corner--the apologists resorted to saying their personal experiences were why they believed in God. I longed for better evidence, but what they presented couldn't save my God damned soul.
Like a burrowing tick, my stubborn faith persisted. I buckled in for some serious soul searching. Many afternoons were spent in deep prayer, longing for any kind of sign--grand or mundane--from the almighty. I eventually took to "testing" him, even though I knew the Bible said not to. At that point, though, I'd rather he reveal himself punishing me than to stop believing.
For one week, I told him I'd stop praying, reading my Bible, and going to church. I just wanted to see how things would change. A week passed, and nothing did. The test extended to two weeks, then three, and... then a month had passed. My life remained very much the same.
Finally, it felt like that tick stopped digging. What little faith I had left was dead. I tried bringing it back to life now and then, but alas, I knew it gasped its last breath years ago.
It hasn't been an entirely smooth ride, but I think losing my religion was the best thing that could have happened to me. It's nice to know my thoughts aren't being monitored, y'know? Some weird shit goes on in my brain. I just wish I'd figured this all out when I was still a kid. It would have spared me a lot of pain.