Since 2000, the number of Spanish-speaking members of the Church of Jesus Christ of Latter-Day Saints in the United States has doubled. Many of those Latino Mormons lack legal documentation, which poses a problem for the church. Young Mormons are called to serve missions — two years on the road, trying to convert strangers to the faith. So what happens when Mormons without papers come of legal age, and set off on their missions? Isaac’s story sheds some light on this quiet demographic. Isaac is his middle name. When his father, Rafael, was 17 years old in Mexico City,...