After waves of barbarians swept through the Roman Empire, Charlemagne struck a deal with Pope Leo III to establish both religious and political stability.
Jews, Christians, and Muslims hold sacred not only some of the same scriptures, but also some of the same sites, which they've fought over for centuries.
Leading up to the Renaissance, as many as three popes simultaneously claimed spiritual and political authority--while dissidents decried their corruption.
Martin Luther's reformist ideas threatened to wreak theological and political havoc, but soon his acolytes had exported Protestant concepts all over Europe.
In the 16th century, as Spain expanded Christianity into the New World, the Catholic Church countered the Reformation with its elite "shock troops," the Jesuits.
Many Protestants persecuted breakaway sects as vigorously as the Catholic Church has attacked heretics. But when some victims sought refuge in America, they too turned tyrannical.
George Whitefield and John Wesley injected emotion into a faith that had become rational, genteel, and--for too many Christians--boring and irrelevant.
By the 19th century, Europe had begun to export its own brand of commerce and Christianity to Africa, while industrialized urban poverty choked religion at home.
It took the Vatican two centuries to accept a heliocentric universe. But science proved only one threat to faith, and change often sparked a fundamentalist reaction.
