I was raised Lutheran and only exposed to other Christians, perhaps mostly Catholics and a few other protestant sects. There was so much dissimilarity between beliefs of the various Christian sects and my own, that like the Muslim surrounded by Christians who goes atheist, the same dynamic was at work for me. Everyone claimed to be right in a sea of mutually exclusive beliefs. I pondered this some, and sort of came up with the understanding that no one really could know which religion was right, and through some principle of logic which I never heard defined until years later, I realized that at best, the majority had to be wrong. And it seemed quite possible that everyone was wrong.
I looked into finding the right religion, but realized along the way, there could be no logical way this could be done. That's only a few steps away to admitting that none of the various religions make viable sense. Then explore the major religions opposed to Christianity, like Islam, Hindu, and Tao, and you're only one step away from throwing the whole thing out. The only logical thing left that makes sense is that they are all irrelevant, and none deserve a commitment of my time.