And that’s one of the most important points to grasp about evangelicalism: It’s not a denomination. It’s not even a single tradition. It is, rather, a complex and fluid movement dedicated to the renewal of Christianity, largely among Protestants, though its efforts have occasionally reached into Catholicism. Its focus is on preaching the euangelion, a New Testament Greek word meaning “good news” or “good message.” The specific contours of that message are often debated: While most scholars of the movement hold to the Bebbington quadrilateral, some think a more complete picture is given by the Larsen pentagon. Those debates can get rather scholastic.

So if you need something a little pithier, here’s the definition that Kidd offers in his new book: “Evangelicals are born-again Protestants who cherish the Bible as the Word of God and who emphasize a personal relationship with Jesus Christ through the Holy Spirit.” It would be difficult to do much better in a single sentence.

That said, the state of evangelicalism in America today is such that a single sentence can’t capture much of the complexity. Thus the full title of Kidd’s book: Who Is an Evangelical? The History of a Movement in Crisis. And Kidd believes, as do I, that the language of “crisis” is appropriate: As he comments late in his book, “The 2016 presidential election would become the most shattering experience for evangelicals since the Scopes Trial.”

Peter Wehner: The deepening crisis in evangelical Christianity

Kidd begins his book with a concise but assured history of the evangelical movement, from its origins in 18th-century England through the 20th century. The Scopes trial—especially as reported by H. L. Mencken’s outraged mockery of William Jennings Bryan’s insistence that Darwinian theory and Christianity are incompatible—established evangelicals in the American public mind as ignorant yahoos who could safely be ignored. (That Mencken had great respect for more thoughtful evangelicals, including the conservative Presbyterian J. Gresham Machen, went unnoticed. It’s instructive to contrast Mencken’s obituary of Bryan with his obituary of Machen.) This general dismissal by journalists and intellectuals lasted until the rise of the self-declared evangelical Jimmy Carter, which led to Time magazine declaring 1976 The Year of the Evangelical.

But this is where the strangest, and perhaps the most consequential, chapter in the history of American evangelicalism began. For in the 1980 election the newly confident evangelical movement, in its self-understanding as the Moral Majority, supported not its co-religionist Jimmy Carter but the divorced former Hollywood actor Ronald Reagan. And that inaugurated the affiliation of white American evangelicals with the Republican Party that has lasted to this day. As Kidd explains:

Forming the Moral Majority freed [Falwell] from tax regulations against direct political advocacy by churches. Unlike [Billy] Graham, Falwell did not begin by seeking access to the top levels of power. Instead, he sought to mobilize fundamentalists and evangelicals to change the occupants of political offices. He told Christians that it was sinful not to vote. Asking pastors to hold voter registration drives, Falwell told them that they needed to get people “saved, baptized, and registered” to vote. The agenda of the Republican evangelical insiders was born.

The precise contours of what happened to evangelicals during the Carter administration are still hotly debated by historians. Certainly abortion rights—which Carter supported and Reagan did not—played a major role, even though that was a recent priority for evangelicals. More generally, the social conservatism of many evangelicals, especially in the South, made them feel less and less at home with the comparatively progressive sexual and racial politics of the Democratic Party. And the fact that Reagan could speak openly of God—in the ’60s, well after his divorce and remarriage, he had had some kind of religious awakening, and became a regular attender of Bel Air Presbyterian Church in Los Angeles—sweetened the pill.