The Catholic Church, also known as the Roman Catholic Church, is the world's largest Christian denomination, with more than a billion members, over half of all Christians and more than one-sixth of the world's population, although the number of practicing as opposed to lapsed Catholics worldwide is not reliably known. It is a communion of the Western, (or Latin Rite) Church, and 22 autonomous Eastern Catholic Churches (called particular churches), comprising a total of 2,795 dioceses in 2008. The Church's highest earthly authority in matters of faith, morality, and governance is the Pope, currently Pope Benedict XVI, who holds supreme authority in concert with the College of Bishops, of which he is the head. The Catholic community is made up of an ordained ministry and the laity; members of either group may belong to organized religious communities.

The Church defines its mission as spreading the gospel of Jesus Christ, administering the sacraments and exercising charity. It operates social programs and institutions throughout the world, including Catholic schools, universities, hospitals, missions and shelters, and the charity confederation Caritas Internationalis.

The Catholic Church believes itself to be the original Church founded by Jesus upon the Apostles, among whom Simon Peter held the position of chief apostle. The Church also believes that its bishops, through apostolic succession, are consecrated successors of these apostles, and that the Bishop of Rome (the Pope) as the successor of Peter, possesses a universal primacy of jurisdiction and pastoral care.

Church doctrines have been defined through various ecumenical councils, following the example set by the first Apostles in the Council of Jerusalem. On the basis of promises made by Jesus to his apostles, described in the Gospels, the Church believes that it is guided by the Holy Spirit and so protected from falling into doctrinal error.

Catholic beliefs are based on the deposit of Faith (containing both the Holy Bible and Sacred Tradition) handed down from the time of the Apostles, which are interpreted by the Church's teaching authority. Those beliefs are summarized in the Nicene Creed and formally detailed in the Catechism of the Catholic Church. Formal Catholic worship is termed the liturgy. The Eucharist is the center of Catholic worship. It is one of seven sacraments which mark key stages in the lives of believers.

With a history spanning almost two thousand years, the Church is "the world's oldest and largest institution" and has played a prominent role in the history of Western civilization since at least the 4th century. In the 11th century, a major split, sometimes called the Great Schism, occurred between Eastern and Western Christianity. Those Eastern churches which remained in, or later re-established, communion with the Pope, form the Eastern Catholic churches and those which remain independent of papal authority are usually known as Orthodox churches. In the 16th century, partly in response to the rise of the Protestant Reformation, the Church engaged in its own process of reform and renewal, known as the Counter-Reformation.

Although the Church maintains that it is the "One, Holy, Catholic and Apostolic Church" founded by Jesus and in which is found the fullness of the means of salvation, it also acknowledges that the Holy Spirit can make use of other Christian communities to bring people to salvation. It believes that it is called by the Holy Spirit to work for unity among all Christians, a movement known as ecumenism.