For many Americans, it was thrilling to be alive in 1919. The end of World War I had brought hundreds of thousands of soldiers home. Cars were rolling off the assembly lines. New forms of music, like jazz, were driving people to dance. And science was in the ascendant, after helping the war effort. Women, having done so much on the home front, were ready to claim the vote, and African-Americans were eager to enjoy full citizenship, at long last. In a word, life was dazzlingly modern.

But for many other Americans, modernity was exactly the problem. As many parts of the country were experimenting with new ideas and beliefs, a powerful counterrevolution was forming in some of the nation’s largest churches and Bible institutes. A group of Christian leaders, anxious about the chaos that seemed to be enveloping the globe, recalibrated the faith and gave it a new urgency. They knew that the time was right for a revolution in American Christianity. In its own way, this new movement — fundamentalism — was every bit as important as the modernity it seemingly resisted, with remarkable determination.

Beginning on May 25, 1919, 6,000 ministers, theologians and evangelists came together in Philadelphia for a weeklong series of meetings. They heard sermons on everything from “Christ and the Present Crisis” to “Why I Preach the Second Coming.” The men and women assembled there believed that God had chosen them to call Christians back to the “fundamentals” of the faith, and to prepare the world for one final revival before Jesus returned to earth. They called their group the World’s Christian Fundamentals Association.

A Minneapolis Baptist preacher named William Bell Riley organized the meetings. A tall, austere and uncompromising man, Riley was a natural-born crusader, who rarely saw a religious fight he did not think he could win. Under his leadership, the event drew participants from all around the county. Contrary to popular stereotypes, the centers of fundamentalism were in the nation’s major northern and western cities — New York, Chicago, Denver, Los Angeles, Seattle — and not the rural South.