The book is devoted to the historical conflict between Roman Catholicism and Protestantism. White writes that the Papacy propagated a corrupt form of Christianity from the time of Constantine I onwards and during the Middle Ages was opposed only by the Waldensians and other small groups, who preserved an authentic form of Christianity. Beginning with John Wycliffe and Jan Huss and continuing with Martin Luther, Huldrych Zwingli, and others, the Reformation led to a partial recovery of biblical truth. In the early 19th century William Miller began to preach that Jesus was about to return to earth; his movement eventually resulted in the formation of the Adventist church.