How did a small Jewish sect become the religion of the Roman Empire? Bart Ehrman, the James A. Gray Distinguished Professor of Religious Studies at the University of North Carolina, is a popular writer on early Christianity (see “How Jesus Became God” in 2015 and “Lost Christianities,” 2005). His latest work, “The Triumph of Christianity: How a Forbidden Religion Swept the World,” surveys the fourth and fifth centuries and analyzes the success of Christianity.