At the height of its power, in the 10th century AD, the dioceses of the Church of the East numbered well over a hundred and stretched from Egypt to China. These dioceses were organised into six interior provinces in Mesopotamia, in the Church's Iraqi heartland, and a dozen or more second-rank exterior provinces. Most of the exterior provinces were located in Iran, Central Asia, India and China, testifying to the Church's remarkable eastern expansion in the Middle Ages. A number of East Syriac dioceses were also established in the towns of the eastern Mediterranean, in Palestine, Syria, Cilicia and Egypt.

Sources

There are few sources for the ecclesiastical organisation of the Church of the East before the Sassanian (Persian) period, and the information provided in martyr acts and local histories such as the Chronicle of Erbil may not always be genuine. The Chronicle of Erbil, for example, provides a list of East Syriac dioceses supposedly in existence by 225. References to bishops in other sources confirm the existence of many of these dioceses, but it is impossible to be sure that all of them had been founded at this early period. Diocesan history was a subject particularly susceptible to later alteration, as bishops sought to gain prestige by exaggerating the antiquity of their dioceses, and such evidence for an early diocesan structure in the Church of the East must be treated with great caution. Firmer ground is only reached with the 4th-century narratives of the martyrdoms of bishops during the persecution of Shapur II, which name several bishops and dioceses in Mesopotamia and elsewhere.

The ecclesiastical organisation of the Church of the East in the Sassanian period, at least in the interior provinces and from the 5th century onwards, is known in some detail from the records of synods convened by the patriarchs Isaac in 410, Yahballaha I in 420, Dadishoʿ in 424, Acacius in 486, Babaï in 497, Aba I in 540 and 544, Joseph in 554, Ezekiel in 576, Ishoʿyahb I in 585 and Gregory in 605.Chabot, 274–5, 283–4, 285, 306–7 and 318–51 These documents record the names of the bishops who were either present at these gatherings, or who adhered to their acts by proxy or later signature. These synods also dealt with diocesan discipline, and throw interesting light on the problems which the leaders of the church faced in trying to maintain high standards of conduct among their widely dispersed episcopate.

After the Arab conquest in the 7th century, the sources for the ecclesiastical organisation of the Church of the East are of a slightly different nature from the synodical acts and historical narratives of the Sassanian period. As far as its patriarchs are concerned, reign-dates and other dry but interesting details have often been meticulously preserved, giving the historian a far better chronological framework for the Umayyad and ʿAbbasid periods than for the Mongol and post-Mongol periods. The 11th-century Chronography of Elijah of Nisibis, edited in 1910 by E. W. Brooks and translated into Latin (Eliae Metropolitae Nisibeni Opus Chronologicum), recorded the date of consecration, length of reign and date of death of all the patriarchs from Timothy I (780–823) to Yohannan V (1001–11), and also supplied important information on some of Timothy's predecessors.

Valuable additional information on the careers of the East Syriac patriarchs is supplied in histories of the Church of the East since its foundation written by the 12th-century East Syriac author Mari ibn Sulaiman and the 14th-century authors ʿAmr ibn Mattai and Sliba ibn Yuhanna. Mari's history, written in the second half of the 12th century, ends with the reign of the patriarch ʿAbdishoʿ III (1139–48). The 14th-century writer ʿAmr ibn Mattai, bishop of Tirhan, abridged Mari's history but also provided a number of new details and brought it up to the reign of the patriarch Yahballaha III (1281–1317). Sliba, in turn, continued ʿAmr's text into the reign of the patriarch Timothy II (1318 – c. 1332). Unfortunately, no English translation has yet been made of these important sources. They remain available only in their original Arabic and in a Latin translation (Maris, Amri, et Salibae: De Patriarchis Nestorianorum Commentaria) made between 1896 and 1899 by Enrico Gismondi.

There are also a number of valuable references to the Church of the East and its bishops in the Chronicon Ecclesiasticum of the 13th-century West Syriac writer Bar Hebraeus. Although principally a history of the Syriac Orthodox Church, the Chronicon Ecclesiasticum frequently mentions developments in the East Syriac church that affected the West Syriacs. Like its East Syriac counterparts, the Chronicon Ecclesiasticum has not yet been translated into English, and remains available only in its Syriac original and in a Latin translation (Bar Hebraeus Chronicon Ecclesiasticum) made in 1877 by its editors, Jean Baptiste Abbeloos and Thomas Joseph Lamy.

Rather less is known about the diocesan organisation of the Church of the East under the caliphate than during the Sassanian period. Although the acts of several synods held between the 7th and 13th centuries were recorded (the 14th-century author ʿAbdishoʿ of Nisibis mentions the acts of the synods of Ishoʿ Bar Nun and Eliya I, for example), most have not survived. The synod of the patriarch Gregory in 605 was the last ecumenical synod of the Church of the East whose acts have survived in full, though the records of local synods convened at Dairin in Beth Qatraye by the patriarch Giwargis in 676 and in Adiabene in 790 by Timothy I have also survived by chance.Chabot, 318–51, 482 and 608 The main sources for the episcopal organisation of the Church of the East during the Umayyad and ʿAbbasid periods are the histories of Mari, ʿAmr and Sliba, which frequently record the names and dioceses of the metropolitans and bishops present at the consecration of a patriarch or appointed by him during his reign. These records tend to be patchy before the 11th century, and the chance survival of a list of bishops present at the consecration of the patriarch Yohannan IV in 900 helps to fill one of the many gaps in our knowledge.MS Paris BN Syr 354, folio 147 The records of attendance at patriarchal consecrations must be used with caution, however, as they can give a misleading impression. They inevitably gave prominence to the bishops of Mesopotamia and overlooked those of the more remote dioceses who were unable to be present. These bishops were often recorded in the acts of the Sassanian synods, because they adhered to their acts by letter.

Two lists in Arabic of East Syriac metropolitan provinces and their constituent dioceses during the ʿAbbasid period have survived. The first, reproduced in Assemani's Bibliotheca Orientalis, was made in 893 by the historian Eliya of Damascus.Assemani, BO, ii. 485–9 The second, a summary ecclesiastical history of the Church of the East known as the Mukhtasar al-akhbar al-biʿiya, was compiled in 1007/8. This history, published by B. Haddad (Baghdad, 2000), survives in an Arabic manuscript in the possession of the Chaldean Church, and the French ecclesiastical historian J. M. Fiey made selective use of it in Pour un Oriens Christianus Novus (Beirut, 1993), a study of the dioceses of the West and East Syriac churches.

A number of local histories of monasteries in northern Mesopotamia were also written at this period (in particular Thomas of Marga’s Book of Governors, the History of Rabban Bar ʿIdta, the History of Rabban Hormizd the Persian, the History of Mar Sabrishoʿ of Beth Qoqa and the Life of Rabban Joseph Busnaya) and these histories, together with a number of hagiographical accounts of the lives of notable holy men, occasionally mention bishops of the northern Mesopotamian dioceses.

Thomas of Marga is a particularly important source for the second half of the 8th century and the first half of the 9th century, a period for which little synodical information has survived and also few references to the attendance of bishops at patriarchal consecrations. As a monk of the important monastery of Beth ʿAbe, and later the secretary of the patriarch Abraham II (832–50), he had access to a wide range of written sources, including the correspondence of the patriarch Timothy I, and could also draw on the traditions of his old monastery and the long memories of its monks. Thirty or forty otherwise unattested bishops of this period are mentioned in the Book of Governors, and it is the prime source for the existence of the northern Mesopotamian diocese of Salakh. A particularly important passage mentions the prophecy of the monastery’s superior Quriaqos, who flourished around the middle of the 8th century, that forty-two of the monks under his care would later become bishops, metropolitans, or even patriarchs. Thomas was able to name, and supply interesting information about, thirty-one of these bishops.Budge, Book of Governors, ii. 444–9

Nevertheless, references to bishops beyond Mesopotamia are infrequent and capricious. Furthermore, many of the relevant sources are in Arabic rather than Syriac, and often use a different Arabic name for a diocese previously attested only in the familiar Syriac form of the synodical acts and other early sources. Most of the Mesopotamian dioceses can be readily identified in their new Arabic guise, but on occasion the use of Arabic presents difficulties of identification.

Parthian period By the middle of the 4th century, when many of its bishops were martyred during the persecution of Shapur II, the Church of the East probably had twenty or more dioceses within the borders of the Sassanian empire, in Beth Aramaye (), Beth Huzaye (), Maishan (), Adiabene (Hdyab, ) and Beth Garmaï (), and possibly also in Khorasan. Some of these dioceses may have been at least a century old, and it is possible that many of them were founded before the Sassanian period. According to the Chronicle of Erbil there were several dioceses in Adiabene and elsewhere in the Parthian empire as early as the end of the 1st century, and more than twenty dioceses in 225 in Mesopotamia and northern Arabia, including the following seventeen named dioceses: Bēṯ Zaḇdai (), Karkhā d-Bēṯ Slōkh (), Kaškar (), Bēṯ Lapaṭ (), Hormīzd Ardašīr (), Prāṯ d-Maišān (), Ḥnīṯā (), Ḥrḇaṯ Glāl (), Arzun (), Beth Niqator (apparently a district in Beth Garmaï), Shahrgard, Beth Meskene (possibly Piroz Shabur, later an East Syriac diocese), Hulwan (), Beth Qatraye (), Hazza (probably the village of that name near Erbil, though the reading is disputed), Dailam and Shigar (Sinjar). The cities of Nisibis () and Seleucia-Ctesiphon, it was said, did not have bishops at this period because of the hostility of the pagans towards an overt Christian presence in the towns. The list is plausible, though it is perhaps surprising to find dioceses for Hulwan, Beth Qatraye, Dailam and Shigar so early, and it is not clear whether Beth Niqator was ever an East Syriac diocese.

Sassanid period