VATICAN CITY — Two-and-a-half years ago, Pope Francis was trying to fill a big job. For decades, that position — heading the powerful office that helps vet and select bishops — had gone to consummate Vatican insiders. But Francis had staked his papacy on expanding the boundaries of the Catholic Church. And he had his eye on a surprise contender: an American-born missionary operating in the coastal plains of Peru, some 6,000 miles from Rome.“You know that I am very happy in Peru,” the missionary, then-Bishop Robert Prevost, recalled telling Francis. “But if you ask me to take on a...