The Roman Catholic Church is the largest branch of the Christian Church in the world, with over 1,000 million members, under the jurisdiction of the pope (the Bishop of Rome). In the early Church the Bishop of Rome, as the successor of St Peter, exercised a limited measure of authority over all Christians. With the East-West Schism of 1054 however, his authority was restricted to the Western Church. Despite this, the Church and papacy reached the zenith of their international power during the Middle Ages.

At the Reformation of the 16th century this authority was diminished by the secession of the Protestant Churches. It was at this time that the term Roman Catholic Church came to be used, initially by Protestants as a somewhat derogatory term to imply that the Roman Church was at most one branch of the Christian Church. But the term was also acceptable to many Roman Catholics, inasmuch as it asserted the primacy of the pope over Christians.

From the mid-16th century the Catholic Church responded to the challenge of Protestantism with the movement known as the Counter-Reformation, which brought various reforms and a tightening of Church discipline (Inquisition). Important developments included the founding (1533) of the Jesuit order, which took a leading role in missionary work, and the convoking of the Council of Trent (1545-63), which determined Catholic teaching, organization, and liturgy for the next 400 years.

During the 18th century the Church came under attack from the sceptical spirit of the Enlightenment and the anticlerical ideology of the French Revolution. Under Pius VII the papacy signed a concordat (1801) with Napoleon, restoring Catholicism in France, but Pius's refusal to support the Continental System against Britain led Napoleon to occupy Rome and the Vatican.

In the 19th century the Church and the papacy reacted defensively to the modern world, responding to challenges to their teaching and authority by stressing strict obedience and uniformity of belief. In Prussia, Bismarck's attempts to subordinate the Church to the state (Kulturkampf) failed because of the passive resistance of the clergy and the Catholic population. Following the reunification of Italy (1870) the pope's temporal powers were restricted to the Vatican. At the same time the First Vatican Council (1869-70) under Pius IX, declared the infallibility of the pope in matters of doctrine. Catholic missionaries continued to promulgate the faith beyond Europe, into Latin America, Asia and Africa.

During World War II Pope Pius XII, faithful to the Lateran Treaties with Italy, retained the strict neutrality of the Church. A second Vatican Council ('Vatican II', 1962-5) was summoned by John XXIII and reconvened by his successor Paul VI; it set out to modernize the Church's teaching, discipline, and organization. Contacts between the Catholic Church, the Eastern Orthodox and Protestant Churches, and other faiths began to be established. In 1967 the Vatican issued a controversial encyclical that formalized the Church's opposition to artificial contraception. Under John Paul II (1978-), a Pole, the global mission of the papacy has been emphasized. John Paul's papacy has also been marked by his resistance to any change in the Church's teaching on contraception, abortion, divorce, homosexuality, and the celibacy of the priesthood. In 1984 the Church concluded with the Italian government a revision of the Lateran Treaties, formalizing the separation of the Church and state. By the year 2000 the majority of Roman Catholics will be living in Latin America, where 'liberation theology', the identification of the priesthood with the poor, became a source of controversy within the Church.

The Roman Catholic Church has an elaborately organized centralized hierarchy of bishops and priests, who must be celibate, under the pope, whose authority is based on the doctrine of the Petrine Succession (i.e., that the popes are the spiritual heirs to the power vested in St Peter by Jesus).

Owing to the paramount authority accorded to the pope, even rulings that are not technically infallible enjoy great authority. The Roman Catholic Church differs from Protestantism in the importance it attaches to tradition, in addition to the authority of the Bible. The importance of the seven sacraments and the celebration of the liturgy have developed as part of the traditions of the Church. The vast majority of worshippers follow the Roman rite, but there are five Eastern-rite groups which accept the authority of the pope. They are the Byzantine, Antiochene, Alexandrian, Chaldean, and Armenian rites, which encompass a number of different churches.

Veneration of the Virgin Mary and the saints is a further aspect of Roman Catholic traditionalism. The centralized authority of the Church has led to a high degree of elucidation of doctrine and belief, such as the different categories of sin and the qualities of purgatory, heaven, and hell.

Despite this traditionalism, the Church has recognized the need for reform and renewal; the liturgy, once universally held in Latin, is now held in the language spoken by the people. Monasticism remains important, but the importance of lay movements such as the South American 'base communities' is increasingly recognized. The Church participates in the ecumenical movement by sending observers to the World Council of Churches. In recent decades the Church has taken a more active role in world events, notably by its sustained opposition to communism in Eastern Europe and through the growth of movements such as liberation theology in the developing world.