I started, as was assumed up top there, from within the church. I was going to church since before I can remember. It was never "prevelant" in our home, but throughout my life I'd been a member of the church off and on. At about the age 16, I "found God" and was baptized into the Church of the Bretheren.
I spent a bit of time in my own head, "rewriting" parts of the Bible into science-speak: 6 day creation, shorthand for 13 billion years of cosmological evolution. Forming dirt into man, short hand for 3 billion years of biological evolution from inanimate earth to simple genetic material, to a single cell, to.. us. The Garden of Eden, an allegory for the psychological development of the evolving human brain, self awareness, morality... I think a lot of Christian do that to some degree.. I did it a lot. And I still approach the Bible this way.
Once I was comfortable enough that I no longer needed to "justify" my Christian beliefs, I turned my attention to learning everything I could about the workings of the human animal, the human mind, and the world at large -- to understand the world of others, to understand those in close relation to me, to understand myself.. Psychology, comparative religion, physics, biology, sociology, anthropology, archeology, geology, philosophy, aesthetics..
I don't even know precisely when it happened, but somewhere in there, the belief that God created man had morphed into an understanding of (or at least, the questions of) why and how "man created God." The Abrahamic God became just another myth. The Bible became nothing more than an artifact, the story of Jesus "The Christ", just a story. No gods, no ghosts, no "spirits", no "soul", no Heaven, no Hell, no rebirth, no reincarnation.. In their place, a secular, humanistic, materialistic -- almost pantheistic -- understanding of my world. One that has served me well (give or take). And one that has made this world and this life increasingly more intelligible the more I look, the older I get (I am 43 as of this writing).
But the subject of "Religion" facinates me..
It's been about 12 years since I first uttered the phrase as a true personal description of my beliefs, "I am an atheist." The church was neither cause nor catalyst. The church had taken me from point A to point B, and I am thankful for the role it played in my life. But in order for me to reach points C, D, and E, it simply became necessary for me to leave it behind. That's all. No regrets, no hard feelings.