I've told the story of my deconversion from Mormonism here, here, and here, but I haven't quite explained yet how I got from there to atheism.
Before my deconversion, I'd already identified atheism as the main alternate possibility (as explained here: If the church weren't true I'd be an atheist, and other things I learned in seminary). Either the church is true or it isn't. Either the spiritual witness is right or it isn't. Since -- if we wipe away what we learned from the spirit -- what's left?
But I was plagued by self-doubt.
Mormonism had convinced me that spiritual witness was valid as evidence for deciding questions about the way the real-world universe functions. So I put a massive amount of heart and effort and prayer into trying to receive that spiritual witness. And several times I managed to generate an emotional/spiritual experience that I hoped was God talking to me.
But "the spirit" always felt sickly and off. Despite what I wrote in my journal about it, I never fully convinced myself that my spiritual experiences weren't wishful thinking and all in my head. That was why I continued to pray fervently for the "testimony" I didn't have, right up to the day of my deconversion epiphany.
But my doubts about my own spiritual experiences didn't extend to doubts about the reality of other people's spiritual experiences. I though my own were possibly just in my head, but I assumed that it was just because I was unworthy to have real spiritual experiences. I believed that other (more righteous) people were receiving actual communication from God.
That was why it threw me for such a loop when I heard from some faithful Mormons say that people in other religions had spiritual experiences similar to those Mormons have (see my deconversion, part 3). My belief in God was ultimately built on the bedrock of believing trusted friends and family when they said they'd talked to Him. When the same trusted individuals admitted that Mormons didn't necessarily have a monopoly on spiritual witness, I hardly knew what to think.
Then, when I had my grand epiphany that the claims of Mormonism are false, I didn't entirely stop believing in other people's spiritual claims. I merely determined that spiritual witness couldn't be used to answer real-world questions or questions about the nature of God. I immediately saw the parallels among all of the myths and miracles claimed by all of mankind's religions, and concluded that all of these details were inventions by people wishing to explain their experiences with the divine.
Thus I became a Deist. I believed that God or gods exist and created the universe and care about people (enough to commune with them), but that the divine powers don't actually intervene or explain anything specific to anyone. That's where I was at when I entered BYU as a freshman.
Sometime during my first year at BYU, I attended a devotional. My ward had invited one of the BYU religion professors to tell us the story of his conversion to Mormonism. He told an amazingly moving story that -- as far as I could tell -- touched everyone in the room, including me. That was the spirit for sure. Since I'd participated in the same spiritual experience with others whose spirituality wasn't in question, I concluded that that must be the real thing, if anything is. I took the experience as evidence of God's love and of the fact that God can communicate through Mormonism just as through any other religion.
But weirdly it was the beginning of the end. As long as I wasn't sure I'd ever received any spiritual witness, I didn't feel qualified to criticize that type of evidence. But once I had some spiritual evidence of my own, I had something concrete to question. And as soon as I started putting some weight on my evidence -- to rely on it for my belief -- the doubts started to squeeze out. That guy is a talented speaker. He's probably given that same devotional hundreds of times. He knows how to tell his story in such a way that it generates an emotional response in his audience. No supernatural explanation required...
Around the same time, someone had posted a cartoon on a door leading to some offices inside the BYU library. I passed the cartoon all the time because the door was along one of the main stairways leading to some of the lower floors of the library. The cartoon was of a stern-looking man (dressed as a scholar) walking down a staircase. I don't remember exactly what was written on the upper few steps -- I think it was a series of things like questioning the literal inerrancy of the Bible -- but I remember what was written on the last three steps: Deism, Agnosticism, Atheism.
I was annoyed by this cartoon because I felt like it was just an attempt to scare people away from doubting or interpreting the slightest thing for themselves. It looked like an obvious swipe against the "liberal" and "Sunstone" Mormons (today's "middle way" people). I thought "Oh, please!! Just because you don't buy the whole enchilada doesn't mean you're on the road to **shudder** atheism."
But the more I passed that cartoon -- and the more I thought about it -- the more I thought, "Well, actually... Maybe this path does lead to atheism..."
I didn't really have a moment of epiphany the way I did with my deconversion from Mormonism. I just gradually started calling myself an agnostic when I wasn't sure anymore. Meanwhile, the evidence for God's existence started looking weaker and weaker.
Then one day I was explaining to an atheist friend (probably my brother) about how I'm an agnostic because I don't claim to have a proof that God doesn't exist.
He then asked me "But which do you think it is? God exists or God doesn't exist?"
Without hesitation I said "I think God doesn't exist."
He laughed and said, "Then you're an atheist! Admit it!"
I thought about it a second and said, "You're right, I'm an atheist."
And I've been an atheist ever since.