Ross Douthat is an Op-Ed columnist for The New York Times. This article is adapted and excerpted from his new book, “Bad Religion: How We Became a Nation of Heretics.”

IN American religious history, Nov. 8, 1960, is generally regarded as the date when the presidency ceased to be the exclusive property of Protestants. But for decades afterward, the election of the Catholic John F. Kennedy looked more like a temporary aberration....

That is, until now. In 2012, we finally have a presidential field whose diversity mirrors the diversity of American Christianity as a whole.

Barack Obama, Mitt Romney and Rick Santorum all identify as Christians, but their theological traditions and personal experiences of faith diverge more starkly than any group of presidential contenders in recent memory. These divergences reflect America as it actually is: We’re neither traditionally Christian nor straightforwardly secular. Instead, we’re a nation of heretics in which most people still associate themselves with Christianity but revise its doctrines as they see fit, and nobody can agree on even the most basic definitions of what Christian faith should mean....