A prince-bishop is a bishop who is also the civil ruler of some secular principality and sovereignty. Thus the principality or prince-bishopric ruled politically by a prince-bishop could wholly or largely overlap with his diocesan jurisdiction, since some parts of his diocese, even the city of his residence, could be exempt from his civil rule, obtaining the status of free imperial city. If the episcopal see is an archbishop, the correct term is prince-archbishop; the equivalent in the regular (monastic) clergy is prince-abbot. A prince-bishop is usually considered an elected monarch.
In the West, with the decline of imperial power from the 4th century onwards in the face of the barbarian invasions, sometimes Christian bishops of cities took the place of the Roman commander, made secular decisions for the city and led their own troops when necessary. Later relations between a prince-bishop and the burghers were invariably not cordial. As cities demanded charters from emperors, kings, or their prince-bishops and declared themselves independent of the secular territorial magnates, friction intensified between burghers and bishops.
In the Byzantine Empire, the still autocratic Emperors passed general legal measures assigning all bishops certain rights and duties in the secular administration of their dioceses, but that was part of a caesaropapist development putting the Eastern Church in the service of the Empire, with its Ecumenical Patriarch almost reduced to the Emperor's minister of religious affairs.
Holy Roman Empire
Bishops had been involved in the government of the Frankish realm and subsequent Carolingian Empire frequently as the clerical member of a duo of envoys styled Missus dominicus, but that was an individual mandate, not attached to the see. Prince-bishoprics were most common in the feudally fragmented Holy Roman Empire, where many were formally awarded the rank of an Imperial Prince Reichsfürst, granting them the immediate power over a certain territory and a representation in the Imperial Diet (Reichstag).
The stem duchies of the German kingdom inside the Empire had strong and powerful dukes (originally, war-rulers), always looking out more for their duchy's "national interest" than for the Empire's. In turn the first Ottonian (Saxon) king Henry the Fowler and more so his son, Emperor Otto I, intended to weaken the power of the dukes by granting loyal bishops Imperial lands and vest them with regalia privileges. Unlike dukes they could not pass hereditary titles and lands to any descendants. Instead the Emperors reserved the implementation of the bishops of their proprietary church for themselves, defying the fact that according to canon law they were part of the transnational Catholic Church. This met with increasing opposition by the Popes, culminating in the fierce Investiture Controversy of 1076. Nevertheless, the Emperors continued to grant major territories to the most important (arch)bishops. The immediate territory attached to the episcopal see then became a prince-diocese or bishopric (Fürstbistum). The German term Hochstift was often used to denote the form of secular authority held by bishops ruling a prince-bishopric with Erzstift being used for prince-archbishoprics.
Emperor Charles IV by the Golden Bull of 1356 confirmed the privileged status of the Prince-Archbishoprics of Mainz, Cologne and Trier as members of the electoral college. At the eve of the Protestant Reformation, the Imperial states comprised 53 ecclesiastical principalities. They were finally secularized in the 1803 German Mediatization upon the territorial losses to France in the Treaty of Lunéville, except for the Mainz prince-archbishop and German archchancellor Karl Theodor Anton Maria von Dalberg, who continued to rule as Prince of Aschaffenburg and Regensburg. With the dissolution of the Holy Roman Empire in 1806, the title finally became defunct. However, in some countries outside of French control, such as in the Austrian Empire (Salzburg, Seckau, and Olomouc) and the Kingdom of Prussia (Breslau), the institution nominally continued, and in some cases was revived; a new, titular type arose.
No less than three of the (originally only seven) prince-electors, the highest order of Reichsfürsten (comparable in rank with the French pairs), were prince-archbishops, each holding the title of Archchancellor (the only arch-office amongst them) for a part of the Empire; given the higher importance of an electorate, their principalities were known as Kurfürstentum ("electoral principality") rather than prince-archbishoprics:
The suffragan-bishoprics of Gurk (established 1070), Chiemsee (1216), Seckau (1218), and Lavant (1225) sometimes used the Fürstbischof title, but never held any reichsfrei territory. The bishops of Vienna (established 1469) and Wiener Neustadt (1469–1785) didn't control any territory, nor did they claim a princely title.
State of the Teutonic Order
Upon the incorporation of the Livonian Brothers of the Sword in 1237, the territory of the Order's State largely corresponded with the Diocese of Riga. Bishop Albert of Riga in 1207 had received the lands of Livonia as an Imperial fief from the hands of German king Philip of Swabia, he however had to come to terms with the Brothers of the Sword. At the behest of Pope Innocent III the Terra Mariana confederation was established, whereby Albert had to cede large parts of the episcopal territory to the Livonian Order. Albert proceeded tactically in the conflict between the Papacy and Emperor Frederick II: in 1225 he reached the acknowledgement of his status as a Prince-Bishop of the Empire, though the Roman Curia insisted on the fact that the Christianized Baltic territories were solely under the suzerainty of the Holy See. By the 1234 Bull of Rieti, Pope Gregory IX stated that all lands acquired by the Teutonic Knights were no subject of any conveyancing by the Emperor.
Within this larger conflict, the continued dualism of the autonomous Riga prince-bishop and the Teutonic Knights led to a lengthy friction. Around 1245 the Papal legate William of Modena reached a compromise: though incorporated into the Order's State, the archdiocese and its suffragan bishoprics were acknowledged with their autonomous ecclesiastical territories by the Teutonic Knights. The bishops pursued the conferment of the princely title by the Holy Roman Emperor to stress their sovereignty. In the original Prussian lands of the Teutonic Order, Willam of Modena established the suffragan bishoprics of Culm, Pomesania, Samland and Warmia. From the late 13th century onwards, the appointed Warmia bishops were no longer members of the Teutonic Knights, a special status confirmed by the bestowal of the princely title by Emperor Charles IV in 1356.
The Bishops of Durham were also territorial prince-bishops, with the extraordinary secular rank of Earl palatine, for it was their duty not only to be head of the large diocese, but also to help protect the Kingdom against the Scottish threat from the north. The title survived the union of England and Scotland into the Kingdom of Great Britain in 1707 until 1836.
Apart from territories formerly within the Holy Roman Empire, no French diocese had a principality of political significance linked to its see.
However, a number of French bishops did hold a noble title, with a tiny territory usually about their seat; it was often a princely title, especially Count. Indeed, six of the twelve original Pairies (the royal vassals awarded with the highest precedence at Court) were episcopal: the Archbishop of Reims, the Bishop of Langres, and the Bishop of Laon held a ducal title, the bishops of Beauvais, Chalôns, and Noyon had comital status.
They were later joined by the Archbishop of Paris, who was awarded a ducal title, but with precedence over the others.
The bishops of Cetinje, Montenegro, who took the place of the earlier secular (Grand) Voivodes in 1516 had a unique position of Slavonic, Orthodox prince-bishops of Montenegro under Ottoman suzerainty. They actually became the secularized, hereditary princes and ultimately Kings of Montenegro in 1852, as reflected in their styles:
- first Vladika i upravitelj Crne Gore i Brda ("Bishop and Ruler of Montenegro and the Highlands")
- from 13 March 1852 (New Style): Po milosti Božjoj knjaz i gospodar Crne Gore i Brda ("By the grace of God Prince and Sovereign of Montenegro and the Highlands")
- from 28 August 1910 (New Style): Po milosti Božjoj kralj i gospodar Crne Gore ("By the grace of God, King and Sovereign of Montenegro")
From 1472 to 1967, the bishop of Coimbra held the comital title of Count of Arganil, being thus called "bishop-count" (Portuguese: Bispo-Conde). The comital title is still held de jure, but since Portugal is a republic and nobility privileges are abolished, its use declined during the 20th century.