Byzantine Papacy in the context of "Pope Constantine"

The history of Italy in the Middle Ages can be roughly defined as the time between the collapse of the Western Roman Empire and the Italian Renaissance. Late antiquity in Italy lingered on into the 7th century under the Ostrogothic Kingdom and the Byzantine Empire under the Justinian dynasty, the Byzantine Papacy until the mid 8th century. The "Middle Ages" proper begin as the Byzantine Empire was weakening under the pressure of the Muslim conquests, and most of the Exarchate of Ravenna finally fell under Lombard rule in 751. From this period, former states that were part of the Exarchate and were not conquered by the Lombard Kingdom, such as the Duchy of Naples, became de facto independent states, having less and less interference from the Eastern Roman Empire.

Lombard rule ended with the invasion of Charlemagne in 773, who established the Kingdom of Italy and the Papal States in large parts of Northern and Central Italy. This set the precedent for the main political conflict in Italy over the following centuries, between the Pope and the Holy Roman Emperor, culminating with conflict between Pope Gregory VII and Henry IV and the latter's "Walk to Canossa" in 1077.

In the year before the First Council of Constantinople in 381, Nicene Christianity became the official religion of the Roman Empire when Theodosius I, emperor of the East, Gratian, emperor of the West, and Gratian's junior co-ruler Valentinian II issued the Edict of Thessalonica in 380, which recognized the catholic orthodoxy, as defined by the Council of Nicea, as the Roman Empire's state religion. Historians refer to this state-sponsored church using a variety of terms: the catholic church, the orthodox church, the imperial church, the Roman church, or the Byzantine church, with some also used for wider communions extending beyond the Roman Empire. The Eastern Orthodox Church, Oriental Orthodoxy, and the Catholic Church all claim to stand in continuity from the Nicene Christian church to which Theodosius granted recognition. Political differences between the Eastern Roman Empire and the Persian Sassanid Empire led to the separation of the Church of the East in 424. A doctrinal split within the imperial church led to the independence of early Oriental Orthodoxy, while the fall of the Western Roman Empire initiated the gradual separation between Eastern and Western Christianity, culminating in the East-West schism of 1054. The Western Church evolved into the Latin Catholic Church while the Eastern Church that remained under the patronage of the Eastern empire and evolved into the Greek Orthodox Church.

Earlier in the 4th century, following the Diocletianic Persecution of 303–313 and the Donatist controversy that arose in consequence, Constantine the Great had convened councils of bishops to define the orthodoxy of the Christian faith and to expand on earlier Christian councils. A series of ecumenical councils convened by successive Roman emperors met during the 4th and the 5th centuries, but Christianity continued to suffer rifts and schisms surrounding the theological and christological doctrines of Arianism, Nestorianism, Miaphysitism, and Dyophysitism. In the 5th century, the Western Roman Empire decayed as a polity; invaders sacked Rome in 410 and in 455, and Odoacer, an Arian barbarian warlord, forced Romulus Augustus, the last nominal Western Emperor, to abdicate in 476. However, apart from the aforementioned schisms, the church as an institution persisted in communion, if not without tension, between the East and West. In the 6th century, the Byzantine armies of the Byzantine Emperor Justinian I recovered Italy and other regions of the Western Mediterranean shore. The Byzantine Empire soon lost most of these gains, but it held Rome, as part of the Exarchate of Ravenna, until 751, a period known in church history as the Byzantine Papacy. The early Muslim conquests of the 7th–9th centuries would begin a process of converting most of the then-Christian world in the Levant, Middle East, North Africa, regions of Southern Italy and the Iberian Peninsula to Islam, severely restricting the reach both of the Byzantine Empire and of its church. Christian missionary activity directed from the capital of Constantinople did not lead to a lasting expansion of the formal link between the church and the Byzantine emperor, since areas outside the Byzantine Empire's political and military control set up their own distinct churches, as in the case of Bulgaria in 919.

Pope Stephen II (Latin: Stephanus II; 714 – 26 April 757) was born a Roman aristocrat and member of the Orsini family. Stephen was the bishop of Rome from 26 March 752 to his death on 26 April 757. Stephen II marks the historical delineation between the Byzantine Papacy and the Frankish Papacy. During Stephen's pontificate, Rome was facing invasion by the Lombards when Stephen II went to Paris to seek assistance from Pepin the Short. Pepin defeated the Lombards and made a gift of land to the pope, eventually leading to the establishment of the Papal States.

Pope Martin I (Latin: Martinus I, Greek: Πάπας Μαρτῖνος; between 590 and 600 – 16 September 655), also known as Martin the Confessor, was the bishop of Rome from 21 July 649 to his death 16 September 655. He had served as Pope Theodore I's ambassador to Constantinople, and was elected to succeed him as pope. He was the only pope when Constantinople controlled the papacy whose election had not awaited imperial mandate. For his strong opposition to Monothelitism, Pope Martin I was arrested by Emperor Constans II, carried off to Constantinople, and ultimately banished to Cherson. He is considered a saint by both the Catholic Church and the Eastern Orthodox Church, as well as the last pope recognised as a martyr.

According to Catholicism, the pope is head of the Catholic Church, a position that spans from the time of Saint Peter to the present day.

In the first three centuries of the Christian era, many of Peter's successors as bishops of Rome are obscure figures, most suffering martyrdom along with members of their flock in periods of persecution. During the Early Church, the bishops of Rome enjoyed no temporal power until the time of Constantine. After the fall of the Western Roman Empire about 476, the medieval papacy was influenced by the temporal rulers of Italy; these periods are known as the Ostrogothic Papacy, Byzantine Papacy, and Frankish Papacy. Over time, the papacy consolidated its territorial claims to a portion of the peninsula known as the Papal States. Thereafter, the role of neighboring sovereigns was replaced by powerful Roman families during the saeculum obscurum, the Crescentii era, and the Tusculan Papacy.

Pope Leo II (c. 01 January 611 – 28 June 683) was the Bishop of Rome from 17 August 682 to his death on 28 June 683. One of the popes of the Byzantine Papacy, he is described by a contemporary biographer as both just and learned. He is commemorated as a saint in the Roman Martyrology.