DIOCESES. – In the ecclesiastical order, this is a territory, well defined in its extent and boundaries, governed by a bishop with ordinary power according to the norms of law (cf. CIC, cann. 215 and 329); it generally takes its name from the place where the cathedral church stands, the mother church of all the others, and where the bishop himself habitually resides. This name derives from Roman public law: in the Constantinian age, in fact, the name d. designated a broad district comprising a certain number of provinces and governed by a vicar. In Christian antiquity, however, the d. of today was commonly called a Church; thus 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. Somewhat later the d. was designated by the name parish, until finally the term d. prevailed exclusively. The origin of the d. goes back to the beginnings of Christian propaganda. While the Palestinian territory was under the direct government of the apostolic college, s. Paul took care, during his journeys through the Hellenic Roman world, to establish Christian communities in urban centers, placing at the head of each a college of presbyters assisted by deacons; he himself, in turn, was assisted by trusted disciples who, when necessary, acted in his place among those communities. S. Paul was certainly not the only great missionary among the Gentiles; s. Peter too, after leaving Palestine and beginning from Antioch, broadened his missionary activity. At their death, the disciples continued their work in the territories for which they had received a special mission, so that the apostolic succession remained uninterrupted. And since the cities had become the chief centers of the faithful, it was there that the leaders of the individual Christian communities took up residence under the name of bishops. In this way, as the propaganda progressively expanded,
each city had its bishop, assisted by a clergy equal in rank and authority to those of the other cities, quite apart from whatever personal prestige might derive 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, the number of d. was also greater, to such an extent that it became necessary to intervene (as in Africa) so that they would not multiply excessively to the detriment of discipline.
It is natural that the boundaries of the d. should generally have coincided with the boundaries of the individual urban territories; and that the bishoprics of smaller centers should preferably have gravitated toward the more important centers, that is, toward the capital city of the province within which they were included and with which they had particular economic or political interests. When Christianity spread in the West into lands inhabited by barbarians, or reconquered regions where the invasions had disrupted every civil order, direct recourse was made to the pope for the establishment of the regular ecclesiastical hierarchy and for the division of the country into d. according to common discipline. At that time the pope carried out this task through legates sent to the area; today, too, he brings it to completion through the Roman dicasteries as soon as solid groups of Catholics have been established in a heretical or pagan country. Indeed, according to the discipline in force, only the Roman Pontiff erects and suppresses d., divides the territory of those that prove too extensive and populous, unites two d. permanently or temporarily under a single bishop, and changes their boundaries by detaching part of the territory and incorporating it into another (cf. CIC, cann. 215-16). The first requirement for the establishment of a d. is that a cathedral church be erected in it, that a sufficient and suitable residence for the bishop be provided in the most important and appropriate 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 under way. Indeed, under the law in force, one cannot conceive of a regularly constituted d. that is not subdivided into parishes. In all d. of ancient foundation there is a college of priests for the liturgical official functions of the cathedral church (Cathedral Chapter: v.) which assists the bishop in governing the d. in circumstances determined by the norms of law. For the handling of current affairs, however, the bishop himself is assisted by a group of officials (vicar general, chancellor, etc.) who constitute the diocesan curia.
In the d. of the missions, it is possible that part of the territory is not divided into parishes but entrusted directly to missionaries as cooperators of the Ordinary.
The individual d. are either directly subject to the Roman Pontiff or grouped into ecclesiastical provinces, in each of which one of the bishops—usually the bishop of the most important, most illustrious, or oldest see—with the title of metropolitan (metropolitan see) or archbishop exercises a special presidency over the others, with authority specifically defined by canon law (v. METROPOLITA). These are called suffragan sees. There are, however, archiepiscopal sees without suffragan d. Titular sees (episcopal and archiepiscopal), on the other hand, are those that now survive only as a historical memory, because they no longer have either a subject Catholic population or an ecclesiastical organization, and sometimes no longer even exist as inhabited places; nevertheless, the Holy See continues to confer them upon prelates, who are thereby invested with the episcopal character. This is done so that they may perform the 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 constituted, or so that they may discharge important offices with greater dignity and authority, such as those of nuncios or apostolic administrators, military bishops, or for other particular and personal reasons.
Today among the Eastern peoples many d. are not only circumscribed within a determinate territory, as in the West, but are also limited to a portion of the population living there, namely, 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 have no jurisdiction over the faithful of the Latin rite living in their territory, and vice versa; Armenian bishops have no jurisdiction over the Melchite or Syriac faithful, etc., who are governed by their own hierarchy.
Although the matter of delimiting d. falls within the exclusive competence of the Church, the Church nevertheless often grants the State some power of intervention (v. CIRCOSTRIZIONI ECCLESIASTICHE).
One of the most notable examples of this in the more recent concordats is contained in Articles 16–17 of the Italian Concordat. By these it was established that the Holy See and Italy should proceed by agreement “through mixed commissions, to a revision of the boundaries of the dioceses, with the aim of making them correspond as far as possible to those of the State’s provinces,” with the dioceses being reduced as they became vacant (for various reasons, this provision saw little practical application), and without suppressing the titles of the dioceses or the chapters, but grouping the dioceses in such a way that their capital cities corresponded to those of the provinces. It was further established that, in any event, no part of the territory subject to the sovereignty of the Italian State should depend upon a bishop whose see was situated in territory subject to the sovereignty of another State; and that no diocese of Italy should include areas of territory subject to the sovereignty of another State. Any modifications that might subsequently be made to the boundaries of the dioceses after the arrangement described above were to be ordered by the Holy See following agreements with the Italian Government and in observance of the same principles, except for minor territorial adjustments required for the good of souls.