It has long been an unwritten rule of the Catholic Church that no citizen of a world superpower can be pope. The informal logic is that such an arrangement would risk putting preeminent temporal and spiritual authority into the hands of the same clan. It is always safer to pick a pope from a more quaint, less powerful country. But at the most unexpected moment, the College of Cardinals broke this understanding by electing the first American pontiff in a line of succession more than 2,000 years old, stretching right back to Christ's selection of St. Peter. When Pope Leo XIV, formerly known as Cardinal Robert Prevost, emerged as the bishop of Rome on May 8, jaws dropped to the cobbled pavement of Vatican City. The Catholic Church is among the oldest thriving institutions in...