Christianity has had an incalculable impact on human history, not just spiritual beliefs and the organization of religion, but in politics, war and human society. Diarmaid MacCullough takes the story of Christianity back to its origins in Judaism and Greek culture a thousand years before Jesus Christ's birth and forward to its expansion in the contemporary world. He explores the ways in which, over three millennia, the cosmic puzzle of God made human gave Christianity a constant struggle to find its identity. He shows how the Roman Empire moved form executing Jesus and persecuting his followers to protecting an established Christian Church; how Rome, the city where Christ's foremost apostles Peter and Paul met their deaths, has come to symbolize one version of the Christian Church. He points to the great might-have-been of Christian history, when, in 451, many Christians rejected an Emperor's imposed compromise solution to the Jesus problem and embarked on ventures to make Christianity a religion of Africa and the Far East. He explores Christianity's complicated and often contentious relationship to its parent Judaism and cousin Islam, and tells the story of the sixteenth-century split within Western Christianity which produced Protestantism and a continuing Roman Catholicism.

In this book we see how Christianity has changed its mind on vital moral questions, such as the permissibility of warfare and slavery, and we learn how the campaign against slavery not only transformed Christianity but helped turn it into a worldwide faith.