The most rapidly growing religious category today is composed of those Americans who say they have no religious affiliation. While middle-age and older Americans continue to embrace organized religion, rapidly increasing numbers of young people are rejecting it.

As recently as 1990, all but 7 percent of Americans claimed a religious affiliation, a figure that had held constant for decades. Today, 17 percent of Americans say they have no religion, and these new "nones" are very heavily concentrated among Americans who have come of age since 1990. Between 25 percent and 30 percent of twentysomethings today say they have no religious affiliation — roughly four times higher than in any previous generation.

So, why this sudden jump in youthful disaffection from organized religion? The surprising answer, according to a mounting body of evidence, is politics. Very few of these new "nones" actually call themselves atheists, and many have rather conventional beliefs about God and theology. But they have been alienated from organized religion by its increasingly conservative politics.

During the 1980s, the public face of American religion turned sharply right. Political allegiances and religious observance became more closely aligned, and both religion and politics became more polarized. Abortion and homosexuality became more prominent issues on the national political agenda, and activists such as Jerry Falwell and Ralph Reed began looking to expand religious activism into electoral politics. Church attendance gradually became the primary dividing line between Republicans and Democrats in national elections.

This political "God gap" is a recent development. Up until the 1970s, progressive Democrats were common in church pews and many conservative Republicans didn't attend church. But after 1980, both churchgoing progressives and secular conservatives became rarer and rarer. Some Americans brought their religion and their politics into alignment by adjusting their political views to their religious faith. But, surprisingly, more of them adjusted their religion to fit their politics.

We were initially skeptical but the evidence convinced us that many Americans now are sorting themselves out on Sunday morning on the basis of their political views. For example, in our Faith Matters national survey of 3,000 Americans, we observed this sorting process in real time, when we interviewed the same people twice, about one year apart.

For Americans, this alignment of religion and politics was divinely ordained, a long-sought retort to the immorality of the 1960s. Other Americans were not so sure.

Throughout the 1990s and into the new century, the increasingly prominent association between religion and conservative politics provoked a backlash among moderates and progressives, many of whom had previously considered themselves religious. The fraction of Americans who agreed "strongly" that religious leaders should not try to influence government decisions nearly doubled from 22 percent in 1991 to 38 percent in 2008, and the fraction who insisted that religious leaders should not try to influence how people vote rose to 45 percent from 30 percent.

This backlash was especially forceful among youth coming of age in the 1990s and just forming their views about religion. Some of that generation, to be sure, held deeply conservative moral and political views, and they felt very comfortable in the ranks of increasingly conservative churchgoers. But a majority of the Millennial generation was liberal on most social issues, and above all, on homosexuality. The fraction of twentysomethings who said that homosexual relations were "always" or "almost always" wrong plummeted from about 75 percent in 1990 to about 40 percent in 2008.