Roman Catholicism, Christian church that has been the decisive spiritual force in the history of Western civilization. Along with Eastern Orthodoxy and Protestantism, it is one of the three major branches of Christianity.

The Roman Catholic Church traces its history to Jesus Christ and the Apostles. Over the course of centuries it developed a highly sophisticated theology and an elaborate organizational structure headed by the papacy, the oldest continuing absolute monarchy in the world.

The number of Roman Catholics in the world (nearly 1.1 billion) is greater than that of nearly all other religious traditions. There are more Roman Catholics than all other Christians combined and more Roman Catholics than all Buddhists or Hindus. Although there are more Muslims than Roman Catholics, the number of Roman Catholics is greater than that of the individual traditions of Shīʿite and Sunni Islam.

These incontestable statistical and historical facts suggest that some understanding of Roman Catholicism—its history, its institutional structure, its beliefs and practices, and its place in the world—is an indispensable component of cultural literacy, regardless of how one may individually answer the ultimate questions of life and death and faith. Without a grasp of what Roman Catholicism is, it is difficult to make historical sense of the Middle Ages, intellectual sense of the works of Thomas Aquinas, literary sense of The Divine Comedy of Dante, artistic sense of the Gothic cathedrals, or musical sense of many of the compositions of Haydn and Mozart.

At one level, of course, the interpretation of Roman Catholicism is closely related to the interpretation of Christianity as such. By its own reading of history, Roman Catholicism originated with the very beginnings of Christianity. An essential component of the definition of any one of the other branches of