"Islam" is surely nearly everyone's answer. As American Christians experiment with ever-milder versions of their faith, Muslims display a fervor for extreme interpretations of Islam. As Europe suffers the lowest population growth rates ever recorded, Muslim countries have some of the highest.
But, argues Philip Jenkins recently in the Atlantic Monthly, Islam is the wrong answer. He shows how Christianity is the religion currently undergoing the most basic rethinking and the largest increase in adherents. He makes a good case for its militancy most affecting the next century.
"For obvious reasons," notes this professor of history and religious studies at Pennsylvania State University, "news reports today are filled with material about the influence of a resurgent and sometimes angry Islam. But in its variety and vitality, in its global reach, in its association with the world's fastest-growing societies, in its shifting centers of gravity, in the way its values and practices vary from place to place . . . it is Christianity that will leave the deepest mark on the 21st century."
What Jenkins dubs the "Christian revolution" is so little noted because Christians divide into two very different regions North (Europe, North America, Australia) and South (South America, Africa, Asia) and we who live in the North only dimly perceive the momentous developments under way in the South. Fortunately, Jenkins is there to guide us.