The Christian Church that acknowledges the pope as its head, especially as this has developed since the Reformation. It has an elaborately organized hierarchy of bishops and priests. Popes are traditionally regarded as successors to St Peter, to whom Christ entrusted his power. In doctrine the Roman Catholic Church is characterized by strict adherence to tradition combined with acceptance of the living voice of the Church and belief in its infallibility. The classic definition of its position was made in response to the Reformation at the Council of Trent (1545–63). During this period the Catholic Church responded to the challenge of Protestantism by the movement known as the Counter-Reformation, which brought about various reforms and a draconian tightening of Church discipline (see inquisition). During the Enlightenment the Church increasingly saw itself as an embattled defender of ancient truth, a belief that culminated in the proclamation of Papal Infallibility in matters of doctrine in 1870. The mid 20th century saw the Church become more open to the world, a change given effect in the decrees of the 2nd Vatican Council (1963–65). However, the pontificates of John Paul II (1978–2005) and Benedict XVI (2005–13) were marked by resistance to any change in the teaching of the Church on the controversial issues of contraception, abortion, divorce, homosexuality, and the celibacy of the priesthood. On all these issues, especially the first, the Church has maintained a position seriously at odds with liberal secular opinion. Benedict XVI was succeeded by Francis, the first Latin American (and the first Jesuit) to be pope. His personal simplicity of style, more liberal views, and ability to communicate on major moral and political issues has boosted the reputation of the Catholic Church.

A historically significant Catholic order is that of the Jesuits or the Society of Jesus, an order of priests founded in 1534 in Paris by Ignatius Loyola, St Francis Xavier, and others. The Society became the spearhead of the Counter-Reformation, though originally intended as a missionary order. The success of Jesuits as missionaries, teachers, scholars, and spiritual directors—as well as the fear they have inspired—manifests how close they have been to their ideal of a disciplined force, effective in the cause of the Roman Church.

Another significant Catholic organization is Opus Dei (Latin, ‘work of God’), which was founded in 1928 by the Spanish priest Josemaria Escrivá de Balaguer (1902–75). Members, of whom there are 90,000 worldwide, may be either priests or lay people, in which case they are encouraged to retain their social position and pursue their profession. Particularly active in General Franco’s Spain (1939–75), the organization has exercised considerable, but controversial, influence on public affairs. There is a separate branch for women, segregation of the sexes being an important principle. Opus Dei emphasizes the austere and conservative aspects of Catholicism; members follow a range of ascetic and spiritual practices, which include daily ‘mortification’ in the form of brief self-flagellation, and celibacy is encouraged. Its secrecy and authoritarianism has been criticized, but Pope John Paul II was a supporter—he beatified de Balaguer in 1992.