In Catholic ecclesiology, a church is an assembly of the faithful, hierarchically ordered, both in the entire world (the Catholic Church), or in a certain territory (a particular church). To be a sacrament (a sign) of the Mystical Body of Christ in the world, a church must have both a head and members (Col. 1:18). The sacramental sign of Christ the head is the sacred hierarchy – the bishops, priests and deacons. More specifically, it is the local bishop, with his priests and deacons gathered around and assisting him in his office of teaching, sanctifying and governing (Mt. 28:19–20; Titus 1:4–9). Thus, the church is fully present sacramentally (by way of a sign) wherever there is a sign of Christ the head, a bishop and those who assist him, and a sign of Christ's body, Christian faithful. Each diocese is therefore considered a particular church. On the worldwide level, the sign of Christ the head is the Pope, and, to be Catholic, particular churches, whether local churches or autonomous ritual churches, must be in communion with this sign of Christ the head, Through this full communion with Saint Peter and his successors the church becomes a universal sacrament of salvation to the end of the age (
The word "church" is applied to the Catholic Church as a whole, which is seen as a single church: the multitude of peoples and cultures within the church, and the great diversity of gifts, offices, conditions and ways of life of its members, are not opposed to the church's unity. In this sense of "church", the list of churches in the Catholic Church has only one member, the Catholic Church itself (comprising Roman and Eastern Churches).
Within the Catholic Church there are local particular churches, of which dioceses are the most familiar form. Other forms include territorial abbacies, apostolic vicariates and apostolic prefectures. The Code of Canon Law states: "Particular Churches, in which and from which the one and only Catholic Church exists, are principally dioceses. Unless the contrary is clear, the following are equivalent to a diocese: a territorial prelature, a territorial abbacy, a vicariate apostolic, a prefecture apostolic and a permanently established apostolic administration." A list of Catholic dioceses, of which on 31 December 2011 there were 2,834, is given at List of Catholic dioceses (alphabetical).
Within the Catholic Church there are also aggregations of local particular churches that share a specific liturgical, theological, spiritual, and canonical heritage, distinguished from other heritages on the basis of cultural and historical circumstances. These are known as autonomous ("sui iuris") churches. The 1990 Code of Canons of the Eastern Churches defines such a church as follows: "A group of Christ's faithful hierarchically linked in accordance with law and given express or tacit recognition by the supreme authority of the Church is in this Code called an autonomous Church." There are 24 such autonomous Catholic churches: One Latin Church (i.e., Western) and 23 Eastern Catholic Churches", a distinction by now more historical than geographical. Although each of them has its own specific heritage, they are all in full communion with the Pope in Rome.
Unlike "families" or "federations" of churches formed through the grant of mutual recognition by distinct ecclesial bodies, the Catholic Church considers itself a single church ("full communion, "one Body") composed of a multitude of particular churches, each of which, as stated, is an embodiment of the fullness of the one Catholic Church. For the particular churches within the Catholic Church, whether autonomous ritual churches (e.g., Coptic Catholic Church, Melkite Catholic Church, Armenian Catholic Church, etc.) or dioceses (e.g., Archdiocese of Birmingham, Archdiocese of Chicago, etc.), are seen as not simply branches, divisions or sections of a larger body. Theologically, each is considered to be the embodiment in a particular place or for a particular community of the one, whole Catholic Church. "It is in these and formed out of them that the one and unique Catholic Church exists."
There are 24 autonomous churches: one Latin Church and twenty-three Eastern Catholic Churches, a distinction by now more historical than geographical. The term sui iuris means, literally, "of its own law", or self-governing. Although all of the particular churches espouse the same beliefs and faith, their distinction lies in their varied expression of that faith through their traditions, disciplines, and canon law. All are in communion with the Holy See.
In Catholic teaching, each diocese (Latin Church term) or eparchy (Eastern term) is also a local or particular church, though it lacks the autonomy of the autonomous churches described above:
A diocese is a section of the People of God entrusted to a bishop to be guided by him with the assistance of his clergy so that, loyal to its pastor and formed by him into one community in the Holy Spirit through the Gospel and the Eucharist, it constitutes one particular church in which the one, holy, catholic and apostolic Church of Christ is truly present and active.
The 1983 Code of Canon Law, which is concerned with the Latin Church alone and so with only one autonomous particular church, uses the term "particular Church" only in the sense of "local Church", as in its Canon 373:
It is within the competence of the supreme authority alone to establish particular Churches; once they are lawfully established, the law itself gives them juridical personality.
The standard form of these local or particular churches, each of which is headed by a bishop, is called a diocese in the Latin Church and an eparchy in the Eastern churches. At the end of 2011, the total number of all these jurisdictional areas (or "sees") was 2,834.