This mammoth book offers a proficient survey of the checkered history of Christianity from its origins to the 21st century. In an engaging voice, journalist Moynahan (The Saint Who Sinned) narrates the story of this upstart Mediterranean religious sect as it developed from a band of ragged disciples with no place to call home to a sophisticated organization with a well-defined priestly hierarchy and often magnificent buildings. He discusses the usual cast of characters from Jesus and Paul to Augustine, Luther, Calvin, Wesley and Pope John Paul II. He argues that the impulse to convert those outside of Christianity is central to the development of the faith, but uses the Crusades and the Spanish Inquisition to demonstrate how this impulse sometimes got out of hand. Moynahan discusses in helpful detail the origins of Islam in the context of the Islamic invasions of Christian Constantinople in the seventh century. However, the book suffers from a lack of balance. Moynahan lavishes attention on Christianity from its beginnings up through the Reformation for the first two-thirds of the book, but then hurries through the establishment of Christianity in America and the development of modern Christianity. Even more perplexing is the complete absence of any examination of Eastern Christianity from its beginnings to the iconoclast crises in the eighth and ninth centuries. In the end, these are minor quibbles with a book that tells crisply, with more than 100 b&w illustrations, a moving tale of the internal and external struggles of Christianity to establish and sustain its religious identity. (Apr.)