Mutual asinine destruction
In reading the stories of people who were raised religious but later abandoned religion, I've noticed an interesting commonality in many (though by no means all) of them. This is that they began to move away from their original religion not so much because of exposure to atheist arguments, but because of some contact with another religion or irrational belief system.
The recent "Why I am an atheist" series of personal testimonies on Pharyngula provides numerous examples. Matthew Donica was raised fundamentalist, started getting disillusioned after exposure to flamboyantly-crazy creationists, and finally read the Iliad and realized that the Bible, like it, was merely another mythical story. Heather V was raised as a Catholic, then became fascinated with Wicca, then married a Baptist and tried to adapt to his beliefs, then married a non-church-goer and, while trying to persuade him to become more "spiritual", came to see the flaws of supernaturalism. Natasha Krasle grew up with a mushy Unitarian-style "spirituality"; after she and her mother joined a church which proceeded to sink into Chopra psychobabble, they became disillusioned with the church and ultimately with spirituality in general. Adam was raised fundamentalist and began to see the light after studying evolution (in order, at first, to learn how to refute it), but what really turned him around was when his parents got interested in fake "alternative medicine" and he realized that religion was just as silly as the nonsense that so exasperated him about that quackery.
I've seen other such stories over the years. The common pattern is obvious: (1) Person grows up religious in the usual way. (2) Person encounters irrational belief system different from his own religion. (3) Person is shocked that anyone could believe such obvious nonsense. (4) Person starts to notice analogies between the newly-encountered nonsense and his own religion's brand of nonsense. (5) Person eventually realizes that both are silly, and abandons religion.
It's not difficult to understand this. Most religious people (not all, but most) absorb their religion when they are too young to think critically, and then maintain it out of inertia in a cultural environment which treats religion as exempt from the kind of logical scrutiny which people are allowed or even expected to apply to other kinds of beliefs, such as political views. They often avoid reading frontal attacks on religion, which the culture's values still consider rude, inappropriate, "just not done".
But in a pluralistic society, those same values that demand respect for one's own religion also demand respect for other religions. The fundamentalist encountering Mormonism, the Muslim discovering Christianity, etc., and being expected to "respect" concepts which strike him as obviously absurd, feels the same kind of cognitive dissonance that we atheists feel on being told we should respect any religion. Why, this is obvious rubbish! But with prolonged exposure, only the most fanatical can avoid noticing parallels between the new rubbish and the old.
Carl Sagan once made a similar observation about intercultural encounters in the ancient world. Greeks and Babylonians, for example, would notice that their respective pantheons of gods claimed similar roles but were otherwise very different. The obvious explanation was that one set of gods had simply been made up by the priests -- but if one set was, why not both?
Magic underwear? God used to be a man? Ancient Hebrews in North America? How can those Mormons believe such drivel? Why can't they just see the plain sensible truth that Jesus died to save us from a force of evil we're all born with because 6,000 years ago a talking snake persuaded a woman made from a rib to eat fruit from a magic tree?
You may have seen this quote from Stephen Roberts: "I contend that we are both atheists. I just believe in one fewer god than you do. When you understand why you dismiss all the other possible gods, you will understand why I dismiss yours." Considering and rejecting an alien religion or superstition forces the theist into precisely this position.
I think this factor has played a major role in the decline of religion which has been going on for centuries and has accelerated in the last century and especially the last ten years. Educated people naturally became aware of belief systems other than their own which existed in distant regions. In a pluralistic society, it's easy to come into some kind of routine contact with different religions. And in the age of the internet, staying strictly within the echo-chamber of homogenous belief takes tremendous effort. Even a person who would indignantly refuse to read an atheist website, even if he lives in a small town in Alabama or Saudi Arabia, can hardly avoid some degree of exposure to other religions.
This is not to deny the importance of frontal attacks on religion such as those of Dawkins, Hitchens, and other New Atheists. Countless people who have abandoned religion have testified that these writings helped them along the path. But for many, it's an encounter with another religion that first sets them on that path.
If this is the case, then religion is truly doomed to further and further decline. The world will not stop getting more and more interconnected. The internet will not go away. Maintaining that echo-chamber of homogenous belief will soon require a total rejection of modernity, to a degree available only to groups like the Amish or the Mormon-fundamentalist cults. Even if we can't destroy religion, the religions, in a cacophony of clashing ideas in billions of brains, will eventually destroy each other.