I also never took the idea of a real interventionist god seriously, and while I've only lived one childhood and so can't compare the effect of this influence to its absence, it might have had to do with the fact that I learned about dead religions at a very early age. By the time my fundamentalist grandmother started proselytizing to me at the age of three, I already had significant exposure to Greek mythology, and I mentally filed "Grandma's beliefs about God" into the same class as "Ancient Greeks' beliefs about gods." It didn't even occur to me until I was about eleven that these were beliefs I was expected to take seriously and have an emotional investment in.
I did experiment with the idea of a non denominational god as a kid, but I never felt the need to make excuses for the hypotheses if they turned up null results. I concluded that if there was a god, it wasn't giving me any reason to worship or believe in it, so I might as well assume it didn't exist.
To break up the awkward silence at the start of a recent Overcoming Bias meetup, I asked everyone present to tell their rationalist origin story - a key event or fact that played a role in their first beginning to aspire to rationality. This worked surprisingly well (and I would recommend it for future meetups).
I think I've already told enough of my own origin story on Overcoming Bias: how I was digging in my parents' yard as a kid and found a tarnished silver amulet inscribed with Bayes's Theorem, and how I wore it to bed that night and dreamed of a woman in white, holding an ancient leather-bound book called Judgment Under Uncertainty: Heuristics and Biases (eds. D. Kahneman, P. Slovic, and A. Tversky, 1982)... but there's no need to go into that again.
So, seriously... how did you originally go down that road?
Added: For some odd reason, many of the commenters here seem to have had a single experience in common - namely, at some point, encountering Overcoming Bias... But I'm especially interested in what it takes to get the transition started - crossing the first divide. This would be very valuable knowledge if it can be generalized. If that did happen at OB, please try to specify what was the crucial "Aha!" insight (down to the specific post if possible).