Third, America’s next geopolitical foe wasn’t a godless state. It was god-fearing, stateless movement: radical Islamic terrorism. A series of bombings and attempted bombings in the 1990s by fundamentalist organizations like Al Qaeda culminated in the attacks of 9/11. It would be a terrible oversimplification to suggest that the fall of the Twin Towers encouraged millions to leave their church, Smith said. But over time, Al Qaeda became a useful referent for atheists who wanted to argue that religion was inherently destructive.

Meanwhile, during George W. Bush’s presidency, Christianity’s association with unpopular Republican policies drove more young liberals and moderates away from both the party and the church. New Atheists, like Christopher Hitchens and Sam Harris, became intellectual celebrities; the 2006 bestseller American Theocracy argued that Evangelicals in the Republican coalition were staging a quiet coup that would plunge the country into disarray and financial ruin. Throughout the Bush presidency, liberal voters—especially white liberal voters— detached from organized religion in ever-higher numbers.

Religion lost its halo effect in the last three decades, not because science drove god from the public square, but rather because politics did. In the 21st century, “not religious” has become a specific American identity—one that distinguishes secular, liberal whites from the conservative, Evangelical right.