DIOCESE. – In ecclesiastical organization, it is that territory, clearly defined in its extent and boundaries, which is governed by a bishop with ordinary power according to the norms of law (cf. CIC, cann. 215 and 329); it usually takes its name from the place where the cathedral church stands, the mother church of all others and the habitual residence of the bishop himself. Such a name derives from Roman public law: in the Constantinian age, in fact, a diocese was designated as a large administrative district, which included a certain number of provinces and was governed by a vicar. In early Christian antiquity, however, the diocese of today was commonly called a Church; so that this term indicated not only the universal Church but also the individual particular Churches, as well as the buildings in which the Christian community gathered. A little later, the diocese was referred to as a parish, until finally the term diocese came to be used exclusively.
The origin of the diocese goes back to the beginnings of Christian missionary activity. While the territory of Palestine was under the direct government of the Apostolic College, St. Paul took care to establish Christian communities in the urban centers during his travels through the Hellenic Roman world, placing at the head of each a college of presbyters assisted by deacons; in turn, he was assisted by trusted disciples who, when necessary, acted in his stead in those communities. St. Paul was certainly not the only great missionary to the Gentiles; St. Peter, too, after leaving Palestine and beginning from Antioch, expanded his missionary activity. After their deaths, their disciples continued their work in the territories for which they had had special responsibility, so that the apostolic succession was not interrupted. And since the cities had become the largest centers of the faithful, here the leaders of the individual Christian communities were established as bishops. In this way, as missionary activity spread, every city had its bishop assisted by a clergy equal in rank and authority to those of other cities, apart from any personal prestige that might accrue to some of these bishops from special gifts of holiness or organizational ability. Hence it came about that in regions where the number of cities or inhabited centers of some importance was greater, so too was the number of dioceses, to the point that it became necessary to intervene (as in Africa) to prevent their excessive multiplication to the detriment of discipline.
It is natural that the boundaries of dioceses should mostly coincide with the boundaries of individual urban territories; and that the bishoprics of smaller centers should gravitate preferentially toward more important centers, that is, toward the provincial capital within which they lay and with which they shared particular economic or political interests. When in the West Christianity spread into lands inhabited by barbarians or reconquered regions where invasions had disrupted every civil order, recourse was had directly to the pope for the establishment of the ecclesiastical region and for the division of the country into dioceses according to common discipline; a task that the pope then carried out by means of legates sent on the spot, and which even today he accomplishes through Roman delegates as soon as solid nuclei of Catholics have been established in a heretical or pagan land. In fact, according to current discipline, only the Roman Pontiff erects and suppresses dioceses, divides the territory of those that are too extensive and populous, permanently or temporarily unites two dioceses under a single bishop, or alters their boundaries by detaching part of the territory to incorporate it into another (cf. CIC, cann. 215-16). The first requirement for the establishment of a diocese is that it have a certain cathedral church, a sufficient and decent residence for the bishop in the most important and suitable place for administration, and that provision be made for the support of the bishop himself and his clergy, that it be possible to train sacred ministers in a local or central seminary, and that the division of the diocesan territory into parishes with clearly defined boundaries be at least begun. For according to current law, a properly constituted diocese cannot be conceived of as not subdivided into parishes. In all dioceses of ancient foundation there is a college of priests for the liturgical service of the cathedral church (Cathedral Chapter: v.) which assists the bishop in the government of the diocese in circumstances determined by the norms of law. But for the handling of current affairs, the bishop is assisted by a group of officials (vicar general, chancellor, etc.) who constitute the diocesan curia.
In missionary dioceses it is possible that part of the territory is not divided into parishes but is entrusted directly to missionaries as co-workers of the Ordinary.
Individual dioceses are either directly subject to the Roman Pontiff or grouped into ecclesiastical provinces, in each of which one of the bishops—usually that of the most important, most illustrious, or oldest see—with the title of metropolitan (metropolitan see) or archbishop holds special presidency over the others with authority specifically defined by canon law (v. METROPOLITANS). These are called suffragan sees. There are, however, archiepiscopal sees without suffragan dioceses. On the other hand, titular sees (of archbishops) are those that now exist only as historical memory, because they no longer have any subject Catholic people or ecclesiastical organization, and sometimes no longer exist even as inhabited places, but which are continued to be conferred by the Holy See on priests who are thereby invested with the episcopal character. This is done so that they may perform sacred functions reserved to bishops, either as auxiliaries to residential bishops or as vicars of the Holy See in territories where the ecclesiastical hierarchy has not yet been regularly established, or so that they may undertake more important offices with greater dignity and authority, such as those of nuncios or apostolic administrators, military ordinaries, or for other particular and personal reasons.
Today among the Eastern Churches many dioceses are not only circumscribed within a definite territory, as in the West, but are also limited to a part of the population living there, namely, to those who follow a particular rite and belong to one of the nationalities that make up the entire population. Thus Catholic bishops of the Byzantine rite do not have jurisdiction over Latin-rite faithful living in their territory, and vice versa; Armenian bishops do not have jurisdiction over Melkite or Syrian faithful, who are governed by their own hierarchy.
Although the matter of the delimitation of dioceses is within the exclusive competence of the Church, it often happens that the Church grants the State some faculty of interference (v. ECCLESIASTICAL CIRCUMSCRIPTIONS).
One of the most notable examples of this to be found in the more recent concordats is contained in articles 16–17 of the Italian concordat. By these articles it was established that the Holy See and Italy should proceed in agreement—“by means of mixed commissions—to a revision of the circumscription of dioceses, with a view to making it, as far as possible, consonant with that of the provinces of the State,” the reduction of dioceses being carried out as they fell vacant (for various reasons, this norm has had little practical application), and without suppressing the titles of the dioceses or their chapters, but grouping the dioceses so that the chief towns of the same should correspond to those of the provinces. It was further established that, in any case, no part of the territory subject to the sovereignty of the Italian State should depend on a bishop whose see lay in territory subject to the sovereignty of another State; and that no Italian diocese should include zones of territory subject to the sovereignty of another State. Any modifications that, after the arrangement just mentioned, might in future be made to the circumscription of dioceses shall be arranged by the Holy See after agreement with the Italian government and in observance of the same principles, save for minor territorial adjustments required by the spiritual welfare of the faithful.