The Catholic Church is by far the largest Christian denomination and the largest organized body of any world religion. Well over a billion people - or over one-sixth of the world's population - belong to the Roman Catholic Church. But what do Catholics actually believe? While not neglecting those aspects of the faith which all Christians share, the author's volume in the "What do we believe?" series pays particular attention to those aspects of the faith which make Catholicism distinctive. She emphasizes the importance to Catholics of a sense of the history of their Church (in both its positive and negative aspects) traceable back to St Peter, the first Pope, and the central role of the Papacy ever since. But more important still is the sacrament of Mass - the sharing in the humanity of God incarnate, the participation - through an act as simple as eating - in his divinity. The title also explains such aspects of the faith as the Holy Spirit, the importance of Mary and other saints, the doctrine of Papal Infallibility, and looks at the challenges the faith has to confront in the twenty-first century.