(cross-posted from a locked livejournal comments thread)
This is a conclusion based on anecdotal evidence, so don't take it seriously, but I suspect most believers achieve their conviction through familiarity with their own belief-system and one or more moments of personal elevation. People expect miracles from their religion: maybe not consciously, maybe they're cynics or convinced of the scientifically explicable nature of the world, but I think almost everybody is at least a little bit open to the idea of the impossible occurring. Then someone has a profound moment, a revelation--uplift, wonder, certainty and happiness in the midst of grief, a random response from a random stranger that was just the thing they needed to hear... and in their mind, they've felt the touch of the divine, and doubt simply falls away. It isn't that the other religions are more or less wrong than theirs; it's that their halting, hesitant faith in their own religion was ultimately rewarded. Even if I find their moment of divine inspiration lacking, the product of a pattern-seeking psyche assigning a religious label to the mundane miracles of the physical world, that doesn't make it any less significant for the person who actually experienced it, does it?
All of which is why I claim to be an atheist and an omnitheist. Does the historical truth of a religion matter, if spirituality and the socio-economic network a church provides is such a positive force in so many peoples' lives? If there is a God that created the world, then it didn't just write the Bible or the Torah or Joseph Smith's golden plates, but rather every action or thought of every living being and all the inanimate matter in the universe. God's message to its creation is its creation, and I can see my own self-consciousness as an argument either for or against the deliberate intentionality of such a message. We're a pattern-seeking life-form, it's true, but is life improved or worsened by rejecting the search? Do the options have to be mutually exclusive?