The United States will never be a truly free, just or tolerant society until an atheist is elected president. (I suppose we could settle for an agnostic.)
Why? Consider the converse: If an atheist cannot be elected president, the United States is not a truly free, just or tolerant society. Any religious test for citizenship or leadership, whether de jure or de facto, is a form of tyranny.
Notice that I did not say, "Until an atheist can be elected president." Political "can be's" are a currency that buys very little in the Wal-Mart voting booth aisle. Blacks have been full de jure citizens of the United States for about 135 years, and it's now been about 40 years since the civil rights movement opened the door for serious talk about how "anyone" could grow up to be president, but there has never been an African-American candidate on a major party national ticket. Similarly, American women have been full members of the polity for almost a century, but there has been only one woman on a major party national ticket, and that ticket was trounced. We will know that a Black, a woman or an atheist truly can be elected president when one actually is elected president.
So, perhaps 2108 isn't far enough away. The point is that the United States, along with other western democracies, is moving (more slowly in our case than in others) toward a bipolar religious world: on one side implacable fundamentalists and other determined dogmatists, and on the other a freethinking coalition of atheists, agnostics and watery, undogmatic believers of various stripes, such as Unitarians, ultraliberal Christians, largely secular Jews, Buddhists, neo-deists and the like. The moderate center will disappear. This does not mean that today's dominant sects will become extinct, only that people who may be nominally, say, Roman Catholics, will gravitate toward one pole or the other. They will become either religious reactionaries like today's fundamentalists, or quasi-humanists having much more in common with unbelievers than with the hardcore faithful.
The process of slow migration toward two opposing positions has been ongoing for centuries. Renaissance, Reformation, Enlightenment; Copernicus, Darwin, Freud. Every chink in the armor of the institutional church simultaneously accomplishes three things: (1) Reduces the pressure on people to be "conventional" church members who bow before social propriety and tradition more than an actual supreme being; (2) Liberates some disgruntled members of the flock, who become free to head for the hills of unbelief; (3) Encourages some worried believers to adopt more hardline positions out of fear that their rock is crumbling.
Today, the migration appears to be accelerating in the United States. Mainline Protestant denominations, such as the Episcopalians, are rapidly approaching schism over the issue of gay rights. In fact, however, the real fissure in these sects is along familiar conservative-liberal lines; even if an accommodation were reached on gay marriage, the groups would tear themselves apart soon enough on another issue similarly pitting the past against the future. Fundamentalists hold more power than ever and are increasingly able to drown out the voices of centrist believers in national debates on "morals" or "values." Nonbelievers, once a tiny, despised minority, are now merely a small, despised minority.
There are two main reasons for the current trend toward increasing polarization. The first is social and pragmatic: Moderation is an ineffective weapon against extremism. Gandhi's policy of nonviolent resistance ousted the British from India because the Brits, though imperialist to the core, were not prepared to launch a genocidal campaign to retain a colony. Similarly, the American civil rights movement succeeded because in 20th century America, for a variety of reasons, firehoses were an acceptable weapon for racist lawmen to use against demonstrators, but machine guns were not. But neither Gandhi nor Martin Luther King would have achieved anything but a speedy execution in Nazi Germany or Stalinist Russia.
In today's America, tolerant religious centrism is the ideological equivalent of passive resistance or civil disobedience. It is effective only if the fundamentalists have little power. Once the reactionaries have achieved a critical mass, centrism wilts. Ripostes such as, "Yes, we should follow God's law, but we shouldn't force it on others," or "True, Jesus said he is the only way to salvation, but he also said to love your neighbor," are useless against adversaries bolstered with the certainty of fundamentalism. Eventually, it becomes easier, and more powerful, to simply declare, "There's no such thing as God's law," or "I don't care what Jesus said and we are not going to run our country on it."
The second reason is more theological: It is logically and emotionally difficult to maintain a somewhat mushy middle position against two adamant extremes. Disbelief is indeed corrosive, as the fundamentalists charge (though this is a good thing, not a bad one). As faith becomes undogmatic, it is often difficult to find a stopping point before reaching agnosticism or vague deism; for many people, the only alternative is to put a stop to the slide by prohibiting all skepticism and enabling dogma to reign supreme. It is a fairly well-known phenomenon that clergy from liberal denominations are sometimes much closer to atheism than their congregations would suspect. Years of study at places like Harvard Divinity School leave such preachers with the conviction that most of the traditional Christian edifice of faith is a mixture of myth, superstition and addled history, just as freethinkers have always maintained. For example, consider the teachings of controversial Bishop John Shelby Spong, author of Why Christianity Must Change or Die and other works. His brand of "Christianity" rejects the divinity of Christ, may even reject the historical reality of Christ, and is just a small step away from asserting that the very notion of "God" is empty. His successors may have the courage to take that final step.
Note that I am not advocating an aggressive campaign to persuade religious moderates to leave behind the remnants of their faith as we prepare for the election of 2108. Each conscience must find its own way in its own time. Barring a catastrophic collapse of western civilization, or a furiously oppressive crackdown by the neo-feudal Bush administration and its fundamentalist partisans, people will continue to move in both directions of their own accord. However, there is certainly no point in Democrats' moving toward the center on religious issues in a misguided attempt to appease the hard right. The center is being slowly deserted; it will eventually become a ghost town. Better to make the freethinking zone as welcoming as possible to the citizens who will inevitably arrive.