Robert Ellwood is the author of over twenty books, ranging from textbooks in religious studies (including Many Peoples Many Faiths, a popular world religion textbook) to books of religious history (The Fifties Spiritual Marketplace, The Sixties Spiritual Awakening, and Islands of the Dawn, among others). He was a professor of world religions at the University of Southern California for thirty years, from 1967 until his retirement in 1997.

Can the histories of the great religious traditions help us to predict their futures? Are broad common patterns discernable within the developments of Hinduism, Chinese religion, Buddhism, Christianity, and Islam? In Cycles of Faith, noted historian of religion Robert Ellwood makes the case that the five largest world religions all move through the stages of Apostolic, Imperial, Devotional, Reformation, and Folk Religion. Cycles of Faith is a completely revised edition of Ellwood's 1988 book, The History and Future of Faith. Ellwood's readable text can provide a useful, theoretical framework to many classes in religious studies.