Christianity and the Church are themes running through almost every aspect of early medieval political, social, and of course, religious history. Though legalized by Constantine and made state religion by Theodosius, Christianity's course from the 330s to 800 was eventful and precarious. The key issues involved A) establishing proper doctrine B); relations among the Roman Papacy, Constantinople, and Western kings; C) elaboration of a church hierarchy, administration, and polity; D) conversion of pagan Europe; and E) the development of monasticism in a way to invigorate the Church as a whole.
The first century of Church history is filled with intellectual dilemmas and doctrinal disputes. The first to emerge has been called Arianism, after the Alexandrian priest Arius who enunciate the doctrine that Christ was "neither co-eternal with nor equal" to God in divinity. In short, Christ was on a lower level. Though condemned by the local bishop, a controversy ensued, necessitating the 325 Council of Nicaea, presided over by Emperor Constantine himself. Arius' ideas were condemned and he was exiled, though he was later restored to his position by Constantine, who himself moved closer to the Arian position by the end of his life. Arianism was more attractive to Easterners, for whom it was more philosophically straightforward. For the next fifty years, Eastern Emperors and their church vacillated regarding doctrine, and only in the 381 First Council of Constantinople was Arianism definitively and finally condemned. It never caught on in the West at all, except for the Goths, who were converted to Christianity by the eastern priest Ulfillias when Arianism was in vogue.
From the 420s, two related controversies particularly wracked Eastern Christianity. Upon becoming Patriarch of Constantinople in 428, the Antiochene Nestorius weighed in on the issue of divine vs. human elements in Christ. For him, there were two natures and two persons in Christ, and no mingling. He was deposed at the Eastern Church's Council of Ephesus (431) for this view, with the strong efforts of the bishop of Alexandria, Cyril, who was supported by Rome. His counter, however, went too far in that it upheld One Nature in One Person. This is the inseparability and indistinguishability of divine and human in Christ. While Nestorius resigned his position, Cyril softened his stand and remained Bishop. That was not the end of either view. Particularly in Syria and other eastern areas, the 'Two Nature' idea survived as Nestorianism. Regarding Cyril's 'One Nature', it was taken up in the 440s by Dioscorus, his successor as Alexandria Bishop, and became known as Monophysitism. The Second Council of Ephesus in 449 followed the Alexandrian approach and established it as orthodoxy, with the Constantinople Bishop being deposed. In 451, Emperor Marcian took an interest in the matter, and held the Council of Chalcedon in 451, at which Monophysitism was condemned again and the idea of Two Natures Coexisting in One Person was promulgated as official doctrine. Though a defeat for the Egyptian clerics, the new orthodoxy never took hold there, and Monophysitism later matured into Coptic Christianity, the majority Egyptian religion until the advent of Islam in the 600s. Furthermore, for the next sixty-five years, successive Eastern Emperors would espouse various views between the two doctrines, with East-West theological agreement not restored until the rule of Emperor Justin I in 518.
These controversies were related to questions of preeminence in the Church itself. By the death of Constantine, there were four primary geographical foci to the Church's hierarchy: Rome, Alexandria (Egypt), Antioch (Syria), and Constantinople. They were all theoretically equal, having been founded by Peter or his legates. It was not long before their archbishops began to assert primacy vis-à-vis each other. From the beginning of the fifth century, Rome began to surge forward. Its chief rival was Constantinople. As the capital of the remnant of the Roman Empire that was officially Christian, the latter city and its Patriarch could already claim significance. Further, in line with precedent set by Constantine, Eastern Emperors had seen their church as an extension of their royal prerogatives, supervising appointments and meddling in doctrine. Part of this entailed insisting upon Constantinople's importance in the Church hierarchy. This was natural, in that Eastern Emperors generally viewed themselves as purposefully Christian sovereigns, with the duty of fighting paganism and upholding right doctrine.
Rome's claim was somewhat different: its bishops, the Popes, came to insist on their See's preeminence and ability to define doctrine for all of Christendom. Relying on Christianity's early sacred history, advocates for Rome, held that St. Peter had been invested with supremacy over the Church by Christ himself. In turn, Peter had been Bishop of Rome, and all Popes were his successors, inheriting his prerogatives and superiority. Cyprian, Bishop of Carthage (248-58) had first articulated such an idea, and Pope Damasus had made it official Western policy in 382, responding to the Constantinople Council of a year earlier, when Rome had not been sufficiently recognized. The idea was that Roman primacy did not rest on any decision of a group of clerics, but on Christ's 'promise' to Peter.
By the 450s, Rome-Constantinople conflicts had grown, as reflected in the doctrinal controversies mentioned above, where something like a Alexandria-Rome axis opposed a Constantinople-Antioch alliance. Part of the growing tension was related to objective circumstances: Pope Leo I could not count on Eastern support when the Huns threatened to sack Rome in 455, and in the absence of any real (Western or Eastern) imperial administration of Rome, the Pope himself was forced to take on several temporal responsibilities, increasing his own sense of esteem. That the senatorial aristocracy of Rome had been impoverished to the point that the Church was much wealthier by comparison strengthened its status as popular patron, and gave basis to the increasingly tenacious claims of Rome's bishops to preeminence. By this time too, Germanic invaders were more prevalent in the West, and revered Rome, not Constantinople. Furthermore, the proliferation of ecclesiastical positions in the West to which people were elected, and about which disputes arose, required a third, impartial party to arbitrate. The Bishop of Rome could do this, raising his status even further. Theologically, Leo built upon earlier Popes' articulations of Petrine doctrine through a legal interpretation based on Roman law whereby an heir inherited a testator's rights and obligations. Thus Peter's authority had been passed on to his successor Clement, who had passed it on continually, to Leo. Leo, then, and his successors had the right to define proper doctrine and lead the Church globally.