I think the thing that made me a seeker-after-rationalism is the same thing that made me an agnostic: Greg Egan's Oceanic.

I grew up in a fundamentalist household and had had one moment of religious euphoria. Oceanic made me confront the fact that religious euphoria, like other euphoria, is just naturalistic phenomena in the brain. Still waiting on my fundamentalist parents to to show evidence for non-naturalistic causes for naturalistic phenomena.