My conversion actually came in stages, though it would not have completed without the supernatural experience at the end.
I was of the high-and-dry, 19th-century style of empirical atheist--I didn't believe in God, but I would if I saw evidence for God. I met my girlfriend (now my wife) in 1979, and she impressed me greatly by being what I thought a Christian should be--peaceful, kind, gentle, pacifist.
We moved into an apartment together in Pittsburgh's East Liberty neighborhood in 1983. At the time East Liberty wasn't the best neighborhood. We were kind of isolated there, and I worried that I was taking her away from any social life, so I suggested she try going to church. We chose Eastminster Presbyterian. I went with Elizabeth to provide security.
The people there were generally welcoming, but I was suspicious of folks who seemed to take things like Noah's Ark seriously. I had serious problems with Christian theology and I often asked people about it. A lady named Robin Williams impressed the heck out of me when I asked her about the Problem of Pain--she simply admitted she didn't know the answer. But she was clearly confident, without ever saying so explicitly, that there was an answer, even if she didn't know what it was. I liked that.
Meanwhile, Elizabeth had introduced me to C.S. Lewis. Although he didn't convert me, he did show me that Christianity was internally self-consistent--that it wasn't absurd or stupid to believe in it. But I still lacked what I considered proof.
In November, 1984, during a very boring sermon, I was checking my watch and wondering if I could sneak out to the church library--they had a '74 Britannica I enjoyed looking through. Without warning, God spoke to me.
It was not a voice--just a sudden conviction that the Christians had it basically right after all, that He was real and somebody I could trust. It felt like remembering something I had forgotten, though I was sure I had never known it in the first place. There was a brief, spurious brightening of my surroundings, and I felt happy--unusual back then, as I had been unemployed most of the year. I was baptized in December.
I can't prove that my experience wasn't some sort of brief dream or subjective event due to brain chemistry. But at various times in my life I have experienced dreams, hallucination (during a high fever), and a drug overdose (in the hospital). This was like none of them. It was intuitively convincing as a revelation from God.
Since then, I have had other supernatural experiences. Even if, God forbid, I had to abandon Christianity for some reason, I could not now go back to being an atheist. I would have to find something else.