Pop worship music. Falling in love with Jesus. Mission trips. Wearing jeans and T-shirts to church. Spiritual searching and church hopping. Faith-based political activism. Seeker-sensitive outreach. These now-commonplace elements of American church life all began as innovative ways to reach young people, yet they have gradually become accepted as important parts of a spiritual ideal for all ages. What on earth has happened?

In The Juvenilization of American Christianity Thomas Bergler traces the way in which, over seventy-five years, youth ministries have breathed new...