First, the old news: I am no longer a person (as I might put it to a believer) blessed with the gift of faith.
The primary fault line for me was one of empathy. The flavor of Christianity I had been given (and, in part, then would prepare for myself) was that of a belief system that was uniquely and universally True, and therefore pointed to the delusion or outright falsehood of other religions. But the contingency of it all -- when I reflected on how as the literal Sweet Talking Son of a Preacher Man I was striving to be a good Christian, but wouldn't an alternate me in the role of Sweet Talking Son of an Imam be trying just as hard to be a good Muslim? - led me to think that it was just terribly unlikely my people got it right and everyone else got it wrong, and this very powerful and loving God let that happen. Empathy (in combination with this deeply instilled idea that Religious Truth must be all encompassing and potentially universal) drove me from my precocious childhood sense of faith.
I'm reading Meghan O'Gieblyn's "God Human Animal Machine: Technology, Metaphor, and the Search for Meaning". She also had a strong Christian background she turned from, and writes
When I was in high school, the pastor of my family's church read the news through the lens of the minor prophets and frequently voiced his opinion, from the pulpit, that Christ would return within his lifetime (he was in his late sixties). For most of my life I had believed that I would live to see the coming of this new age; that my body would be transformed, made immortal, and I would ascend into the clouds to spend eternity with God.That kind of thinking sounded very familiar to me. But now I'm thinking... hasn't at least one flavor of Christians been saying over and over and over for centuries? It feels like a certain gullibility there - akin to believing a tenant saying "oh, THIS month I'll get you the rent, I swear" or that any week now you'll win the lottery.
So that, too, lacks a certain kind of empathy. It's like the modern day believer, putting aside that Matthew 24:36 talk ("But of that day and hour knoweth no man, no, not the angels of heaven, but my Father only.") says - "C'mon... just look at the state of the world, the return of Israel, all that jazz. It's GOTTA be SOON!" and to the extent they acknowledge that line of thinking goes way, way back, it's like they're saying "What a bunch of rubes!" to those earlier generations of believers.
And I do resent that apocalyptic kind of thinking, which scared the bejeebers out of me as a kid, and has distorted our political and economic policy for centuries. It's tough to work to be really good long term planners when you think the end is nigh - ties in with our cult of individualism to make a country of "got mine, forget you" whether the "you" is others now, or in the future.
I do understand that this doesn't have to be the basis of Christian faith - here's a Baptist News piece from 2009 about surveys saying 40-60% of Americans say differing religions can lead to eternal life. (Heh, not even going to get into the eternal life bit.)
So not even every Christian was raised with this sense of uniqueness, and I'd say the many-path approach has a lot more wisdom, which is why I'm affiliated with liberal Unitarian Universalists.
It's funny, I always distrust any faith based on trusting an un-interrogatable "special revelation", but I also realize that I have no mechanism for absolutely saying that ISN'T how the Universal Truth might work. For all I know, one sect has it exactly right, and God is secretly blessing that one group, and the devil to all the rest. It just doesn't seem particularly likely to me.