It’s been said that saints often come in pairs. Sts. Peter and Paul, Mary and Joseph, Francis and Clare, and Louis and Zelie Martin are just a handful of such saints, coupled together through marriage or friendship. Perhaps the best-known modern saintly pair of friends would be Mother Teresa and John Paul II, whose lives intersected many times during her time as Mother Superior of the Missionaries of Charity, and his pontificate. When John Paul II came to visit Mother Teresa’s home in the heart of the slums in Kolkata in 1986, Mother Teresa called it “the happiest day of...