The fourth century of our common era began and ended with a miracle. Traditionally, in the year 312, the Roman emperor Constantine experienced a "vision of the Cross" that led him to convert to Christianity and to defeat his last rival to the imperial throne; and, in 394, a divine wind carried the emperor Theodosius to victory at the battle of the Frigidus River. Other stories heralded the discovery of the True Cross by Constantine's mother, Helena,...