State church of the Roman Empire

Monogramme of Christ (the Chi Rho) on a plaque of a sarcophagus, 4th-century AD, marble, Musei Vaticani, on display in a temporary exhibition at the Colosseum in Rome, Italy

Before the end of the 1st century, the Roman authorities recognized Christianity as a separate religion from Judaism. The distinction, perhaps already made in practice at the time of the Great Fire of Rome in the year 64, was given official status by the emperor Nerva around the year 98 by granting Christians exemption from paying the Fiscus Iudaicus, the annual tax upon the Jews. Pliny the Younger, when propraetor in Bithynia in 103, assumes in his letters to Trajan that because Christians do not pay the tax, they are not Jews.[22][23][24]

Since paying taxes had been one of the ways that Jews demonstrated their goodwill and loyalty toward the empire, Christians had to negotiate their own alternatives to participating in the imperial cult. Their refusal to worship the Roman gods or to pay homage to the emperor as divine resulted at times in persecution and martyrdom.[22][23][24] Church Father Tertullian, for instance, attempted to argue that Christianity was not inherently treasonous, and that Christians could offer their own form of prayer for the well-being of the emperor.[25]

Christianity spread especially in the eastern parts of the empire and beyond its border; in the west it was at first relatively limited, but significant Christian communities emerged in Rome, Carthage, and other urban centers, becoming by the end of the 3rd century, the dominant faith in some of them. Christians accounted for approximately 10% of the Roman population by 300, according to some estimates.[26] According to Will Durant, the Christian Church prevailed over paganism because it offered a much more attractive doctrine and because the church leaders addressed human needs better than their rivals.[27]

In 301, the Kingdom of Armenia, nominally a Roman client kingdom but ruled by a Parthian dynasty,[28] became the first nation to adopt Christianity as its state religion.

In 311, with the Edict of Serdica the dying Emperor Galerius ended the Diocletianic Persecution that he is reputed to have instigated, and in 313, Emperor Constantine issued the Edict of Milan, granting to Christians and others "the right of open and free observance of their worship".[38]

Constantine began to utilize Christian symbols such as the Chi Rho early in his reign but still encouraged traditional Roman religious practices including sun worship. In 330, Constantine established the city of Constantinople as the new capital of the Roman Empire. The city would gradually come to be seen as the intellectual and cultural center of the Christian world.[39]

Over the course of the 4th century the Christian body became consumed by debates surrounding orthodoxy, i.e. which religious doctrines are the correct ones. In the early 4th century, a group in North Africa, later called Donatists, who believed in a very rigid interpretation of Christianity that excluded many who had abandoned the faith during the Diocletianic Persecution, created a crisis in the western empire.[40]

A synod was held in Rome in 313, followed by another in Arles in 314. These synods ruled that the Donatist faith was heresy and, when the Donatists refused to recant, Constantine launched the first campaign of persecution by Christians against Christians, and began imperial involvement in Christian theology. However, during the reign of Emperor Julian the Apostate, the Donatists, who formed the majority party in the Roman province of Africa for 30 years,[41] were given official approval.[42]

Icon depicting Constantine and the bishops of the Council of Nicaea (325). The centrally placed and haloed Emperor holds the Creed of the First Council of Constantinople (381).

Christian scholars and populace within the empire were increasingly embroiled in debates regarding christology (i.e., regarding the nature of the Christ). Opinions ranged from belief that Jesus was entirely human to belief that he was entirely divine. The most persistent debate was that between the homoousian view (the Father and the Son are of one substance), defined at the Council at Nicaea in 325 and later championed by Athanasius of Alexandria, and the Arian view (the Father and the Son are similar, but the Father is greater than the Son). Emperors thereby became ever more involved with the increasingly divided early Church.[43]

Constantine backed the Nicene Creed of Nicaea, but was baptized on his deathbed by the Eusebius of Nicomedia, a bishop with Arian sympathies. His successor Constantius II supported Arian positions: under his rule, the Council of Constantinople in 360 supported the Arian view. After the interlude of Emperor Julian, who wanted to return to the pagan Roman/Greek religion, the west stuck to the Nicene Creed, while Arianism or Semi-Arianism was dominant in the east (under Emperor Valens), until Emperor Theodosius I called the Council of Constantinople in 381, which reasserted the Nicene view and rejected the Arian view. This council further refined the definition of orthodoxy, issuing the Nicene-Constantinopolitan Creed.

Missorium of Emperor Theodosius I, who declared Nicene Christianity to be the State religion of the Roman Empire. This piece was found in Almendralejo.

On 27 February of the previous year, Theodosius I established, with the Edict of Thessalonica, the Christianity of the First Council of Nicaea as the official state religion, reserving for its followers the title of Catholic Christians and declaring that those who did not follow the religion taught by Pope Damasus I of Rome and Pope Peter of Alexandria were to be called heretics:[44]

It is our desire that all the various nations which are subject to our Clemency and Moderation, should continue to profess that religion which was delivered to the Romans by the divine Apostle Peter, as it has been preserved by faithful tradition, and which is now professed by the Pontiff Damasus and by Peter, Bishop of Alexandria, a man of apostolic holiness. According to the apostolic teaching and the doctrine of the Gospel, let us believe in the one deity of the Father, the Son and the Holy Spirit, in equal majesty and in a holy Trinity. We authorize the followers of this law to assume the title of Catholic Christians; but as for the others, since, in our judgment they are foolish madmen, we decree that they shall be branded with the ignominious name of heretics, and shall not presume to give to their conventicles the name of churches. They will suffer in the first place the chastisement of the divine condemnation and in the second the punishment of our authority which in accordance with the will of Heaven we shall decide to inflict.

— Edict of Thessalonica

In 391, Theodosius closed all the "pagan" (non-Christian and non-Jewish) temples and formally forbade pagan worship.[45]

Changes in extent of the Empire ruled from Constantinople.
476 End of the Western Empire; 550 Conquests of Justinian I; 717 Accession of Leo the Isaurian; 867 Accession of Basil I; 1025 Death of Basil II; 1095 Eve of the First Crusade; 1170 Under Manuel I; 1270 Under Michael VIII Palaiologos; 1400 Before the fall of Constantinople

At the end of the 4th century the Roman Empire had effectively split into two parts although their economies and the imperial-recognized church were still strongly tied. The two halves of the empire had always had cultural differences, exemplified in particular by the widespread use of the Greek language in the Eastern Empire and its more limited use in the West (Greek, as well as Latin, was used in the West, but Latin was the spoken vernacular).

By the time Christianity became the state religion of the empire at the end of the 4th century, scholars in the West had largely abandoned Greek in favor of Latin. Even the Church in Rome, where Greek continued to be used in the liturgy longer than in the provinces, abandoned Greek.[lower-alpha 4] Jerome's Vulgate had begun to replace the older Latin translations of the Bible.

The Hagia Sophia basilica in Constantinople, for centuries the largest church building in the world.

The 5th century would see the further fracturing of Christendom. Emperor Theodosius II called two synods in Ephesus, one in 431 and one in 449, the first of which condemned the teachings of Patriarch Nestorius of Constantinople, while the second supported the teachings of Eutyches against Archbishop Flavian of Constantinople.[47]

Nestorius taught that Christ's divine and human nature were distinct persons, and hence Mary was the mother of Christ but not the mother of God. Eutyches taught on the contrary that there was in Christ only a single nature, different from that of human beings in general. The First Council of Ephesus rejected Nestorius' view, causing churches centered around the School of Edessa, a city at the edge of the empire, to break with the imperial church (see Nestorian schism).[47]

Persecuted within the Roman Empire, many Nestorians fled to Persia and joined the Sassanid Church (the future Church of the East). The Second Council of Ephesus upheld the view of Eutyches, but was overturned two years later by the Council of Chalcedon, called by Emperor Marcian. Rejection of the Council of Chalcedon led to the exodus from the state church of the majority of Christians in Egypt and many in the Levant, who preferred Miaphysite theology.[47]

Thus, within a century of the link established by Theodosius between the emperor and the church in his empire, it suffered a significant diminishment. Those who upheld the Council of Chalcedon became known in Syriac as Melkites, the imperial group, followers of the emperor (in Syriac, malka).[48] This schism resulted in an independent communion of churches, including the Egyptian, Syrian, Ethiopian and Armenian churches, that is today known as Oriental Orthodoxy.[49] In spite of these schisms, however, the Chalcedonian Nicene church still represented the majority of Christians within the by now already diminished Roman Empire.[50]

Odoacer's kingdom in 480, after annexing Dalmatia and most of Sicily.

In the 5th century, the Western Empire rapidly decayed and by the end of the century was no more. Within a few decades, Germanic tribes, particularly the Goths and Vandals, conquered the western provinces. Rome was sacked in 410 and 455, and was to be sacked again in the following century in 546.[37]

By 476, the Germanic chieftain Odoacer had conquered Italy and deposed the last western emperor, Romulus Augustus, though he nominally submitted to the authority of Constantinople. The Arian Germanic tribes established their own systems of churches and bishops in the western provinces but were generally tolerant of the population who chose to remain in communion with the imperial church.[37]

In 533, Roman Emperor Justinian in Constantinople launched a military campaign to reclaim the western provinces from the Arian Germans, starting with North Africa and proceeding to Italy. His success in recapturing much of the western Mediterranean was temporary. The empire soon lost most of these gains, but held Rome, as part of the Exarchate of Ravenna, until 751.

Justinian definitively established Caesaropapism,[51] believing "he had the right and duty of regulating by his laws the minutest details of worship and discipline, and also of dictating the theological opinions to be held in the Church".[8] According to the entry in Liddell & Scott, the term orthodox first occurs in the Codex Justinianus: "We direct that all Catholic churches, throughout the entire world, shall be placed under the control of the orthodox bishops who have embraced the Nicene Creed."[52]

By the end of the 6th century the church within the Empire had become firmly tied with the imperial government,[53] while in the west Christianity was mostly subject to the laws and customs of nations that owed no allegiance to the emperor.[12]

A map of the five patriarchates in the Eastern Mediterranean as constituted by Justinian I. Rome is coloured in pink, Constantinople in green, Antioch in blue, Jerusalem in pink and Alexandria in yellow. Leo III extended the jurisdiction of Constantinople to the territories bordered in pink.

Emperor Justinian I assigned to five sees, those of Rome, Constantinople, Alexandria, Antioch and Jerusalem, a superior ecclesial authority that covered the whole of his empire. The First Council of Nicaea in 325 reaffirmed that the bishop of a provincial capital, the metropolitan bishop, had a certain authority over the bishops of the province.[54] But it also recognized the existing supra-metropolitan authority of the sees of Rome, Alexandria and Antioch,[55] and granted special recognition to Jerusalem.[56][57][58]

Constantinople was added at the First Council of Constantinople (381)[59] and given authority initially only over Thrace. By a canon of contested validity,[60] the Council of Chalcedon (451) placed Asia and Pontus,[61] which together made up Anatolia, under Constantinople, although their autonomy had been recognized at the council of 381.[62][63]

Rome never recognized this pentarchy of five sees as constituting the leadership of the church. It maintained that, in accordance with the First Council of Nicaea, only the three "Petrine" sees of Rome, Alexandria and Antioch had a real patriarchal function.[64] The canons of the Quinisext Council of 692, which gave ecclesiastical sanction to Justinian's decree, were also never fully accepted by the Western Church.[65]

Early Muslim conquests of the territories of the patriarchates of Alexandria, Antioch and Jerusalem, most of whose Christians were in any case lost to the orthodox church since the aftermath of the Council of Chalcedon, left in effect only two patriarchates, those of Rome and Constantinople.[66] In 732, Emperor Leo III's iconoclast policies were resisted by Pope Gregory III. The Emperor reacted by transferring to the ecclesiastical jurisdiction of Constantinople in 740 the territories in Greece, Illyria, Sicily and Calabria that had been under Rome (see map), leaving the bishop of Rome with only a minute part of the lands over which the empire still had control.[67]

The Patriarch of Constantinople had already adopted the title of "ecumenical patriarch", indicating what he saw as his position in the oikoumene, the Christian world ideally headed by the emperor and the patriarch of the emperor's capital.[68][69] Also under the influence of the imperial model of governance of the state church, in which "the emperor becomes the actual executive organ of the universal Church",[70] the pentarchy model of governance of the state church regressed to a monarchy of the Patriarch of Constantinople.[70][71]

A map of Muslim expansion in the 7th and 8th centuries.

The Rashidun conquests began to expand the sway of Islam beyond Arabia in the 7th century, first clashing with the Roman Empire in 634. That empire and the Sassanid Persian Empire were at that time crippled by decades of war between them. By the late 8th century the Umayyad caliphate had conquered all of Persia and much of the Byzantine territory including Egypt, Palestine, and Syria.

Suddenly, much of the Christian world was under Muslim rule. Over the coming centuries the successive Muslim states became some of the most powerful in the Mediterranean world.

Though the Byzantine church claimed religious authority over Christians in Egypt and the Levant, in reality the majority of Christians in these regions were by then miaphysites and members of other sects. The new Muslim rulers, in contrast, offered religious tolerance to Christians of all sects. Additionally subjects of the Muslim Empire could be accepted as Muslims simply by declaring a belief in a single deity and reverence for Muhammad (see shahada). As a result, the peoples of Egypt, Palestine and Syria largely accepted their new rulers and many declared themselves Muslims within a few generations. Muslim incursions later found success in parts of Europe, particularly Spain (see Al-Andalus).[72]

The spread of Christianity in Europe by 325 AD (dark blue) and 600 AD (light blue).

During the 9th century, the Emperor in Constantinople encouraged missionary expeditions to nearby nations including the Muslim caliphate, and the Turkic Khazars. In 862 he sent Saints Cyril and Methodius to Slavic Great Moravia. By then most of the Slavic population of Bulgaria was Christian and Tsar Boris I himself was baptized in 864. Serbia was accounted Christian by about 870.[73] In early 867 Patriarch Photios I of Constantinople wrote that Christianity was accepted by the Kievan Rus', which however was definitively Christianized only at the close of the following century.

The spread of Christianity in Europe by 1000.

Of these, the Church in Great Moravia chose immediately to link with Rome, not Constantinople: the missionaries sent there sided with the Pope during the Photian Schism (863–867).[74] After decisive victories over the Byzantines at Acheloos and Katasyrtai, Bulgaria declared its church autocephalous and elevated it to the rank of patriarchate, an autonomy recognized in 927 by Constantinople,[75][76] but abolished by Emperor Basil II Bulgaroktonos (the Bulgar-Slayer) after his 1018 conquest of Bulgaria.

In Serbia, which became an independent kingdom in the early 13th century, Stephen Uroš IV Dušan, after conquering a large part of Byzantine territory in Europe and assuming the title of Tsar, raised the Serbian archbishop to the rank of patriarch in 1346, a rank maintained until after the fall of the Byzantine Empire to the Turks. No Byzantine emperor ever ruled Russian Christendom.

Expansion of the church in western and northern Europe began much earlier, with the conversion of the Irish in the 5th century, the Franks at the end of the same century, the Arian Visigoths in Spain soon afterwards, and the English at the end of the 6th century. By the time the Byzantine missions to central and eastern Europe began, Christian western Europe, in spite of losing most of Spain to Islam, encompassed Germany and part of Scandinavia, and, apart from the south of Italy, was independent of the Byzantine Empire and had been almost entirely so for centuries.

This situation fostered the idea of a universal church linked to no one particular state.[17] Long before the Byzantine Empire came to an end, Poland also, Hungary and other central European peoples were part of a church that in no way saw itself as the empire's church and that, with the East-West Schism, had even ceased to be in communion with it.

The coronation of Charlemagne as emperor.

With the defeat and death in 751 of the last Exarch of Ravenna and the end of the Exarchate, Rome ceased to be part of the Byzantine Empire. Forced to seek protection elsewhere,[77] the popes turned to the Franks and, with the coronation of Charlemagne by Pope Leo III on 25 December 800, transferred their political allegiance to a rival Roman emperor. Disputes between the see of Rome, which claimed authority over all other sees, and that of Constantinople, which was now without rival in the empire, culminated perhaps inevitably[78] in mutual excommunications in 1054.

Communion with Constantinople was broken off by European Christians with the exception of those ruled by the empire (including the Bulgarians and Serbs) and of the fledgling Kievan or Russian Church, then a metropolitanate of the patriarchate of Constantinople. This church became independent only in 1448, just five years before the extinction of the empire,[79] after which the Turkish authorities included all their Orthodox Christian subjects of whatever ethnicity in a single millet headed by the Patriarch of Constantinople.

The Westerners who set up Crusader states in Greece and the Middle East appointed Latin (Western) patriarchs and other hierarchs, thus giving concrete reality and permanence to the schism.[80][81][82] Efforts were made in 1274 (Second Council of Lyon) and 1439 (Council of Florence) to restore communion between East and West, but the agreements reached by the participating eastern delegations and by the emperor were rejected by the vast majority of Byzantine Christians.

In the East, the idea that the Byzantine emperor was the head of Christians everywhere persisted among churchmen as long as the empire existed, even when its actual territory was reduced to very little. In 1393, only 60 years before the fall of the capital, Patriarch Antony IV of Constantinople wrote to Basil I of Muscovy defending the liturgical commemoration in Russian churches of the Byzantine emperor on the grounds that he was "emperor (βασιλεύς) and autokrator of the Romans, that is of all Christians".[19] According to Patriarch Antony, "it is not possible among Christians to have a Church and not to have an emperor. For the empire and the Church have great unity and commonality, and it is not possible to separate them",[83][84][20] and "the holy emperor is not like the rulers and governors of other regions".[20][85]