Christianity in the 4th century was dominated in its early stage by Constantine the Great and the First Council of Nicaea of 325, which was the beginning of the period of the First seven Ecumenical Councils (325–787), and in its late stage by the Edict of Thessalonica of 380, which made Nicene Christianity the state church of the Roman Empire.
Christian persecutions
{| class="wikitable" style="float:right; margin:0 0 0.5em 1em;" border="1" |+ Major communions ofthe 4th-5th centuries ! Communion !! Primary centers |- | Roman Empire andwestern Europe || Rome, Alexandria,Constantinople |- | Church of the East || Syria, Sasanian (Persia) EmpireO'Leary (2000), pp. 131–137. |- | Oriental Orthodox orNon-Chalcedonian || Armenia, Syria, EgyptPrice (2005), pp. 52–55. |- | Donatist Church || North AfricaDwyer (1998), pp. 109–111. |- | Gothic Arian Church || Gothic tribesAnderson (2010), p. 604.Amory (), pp. 259–262. |} With Christianity the dominant faith in some urban centers, Christians accounted for approximately 10% of the Roman population by 300, according to some estimates.Hopkins(1998), p. 191 Roman Emperor Diocletian launched the bloodiest campaign against Christians that the empire had witnessed. The persecution ended in 311 with the death of Diocletian. The persecution ultimately had not turned the tide on the growth of the religion,Irvin (2002), p. 161. and because of the rapid growth, Christians accounted for 56.5% of the Roman population by 350. Christians had already organized to the point of establishing hierarchies of bishops. In 301 the Kingdom of Armenia became the first nation to adopt Christianity, followed by the Georgians in 324, the Axumite Empire circa 328 and the Romans in 380.
Roman Empire
Under Galerius In April 311, Galerius, who had previously been one of the leading figures in the persecutions, issued an edict permitting the practice of the Christian religion under his rule.Lactantius, De Mortibus Persecutorum ("On the Deaths of the Persecutors") ch.34–35 From 313 to 380, Christianity enjoyed the status of being a legal religion within the Roman Empire. It had not become the sole authorized state religion, although it gradually gained prominence and stature within Roman society. After halting the persecutions of the Christians, Galerius reigned for another 2 years. He was then succeeded by an emperor with distinctively pro-Christian leanings, Constantine the Great.
Constantine I Christian sources record that Constantine experienced a dramatic event in 312 at the Battle of Milvian Bridge, after which Constantine claimed the emperorship in the West. According to these sources, Constantine looked up to the sun before the battle and saw a cross of light above it, and with it the Greek words "" ("by this, conquer!", often rendered in the Latin "in hoc signo vinces"); Constantine commanded his troops to adorn their shields with a Christian symbol (the Chi-Ro), and thereafter they were victorious.Gerberding, p.55cf. Eusebius, Life of Constantine How much Christianity Constantine adopted at this point is difficult to discern; most influential people in the empire, especially high military officials, were still pagan, and Constantine's rule exhibited at least a willingness to appease these factions.
The conversion to Christianity
The accession of Constantine was a turning point for the Christian Church. In 313, Constantine issued the Edict of Milan affirming the tolerance of Christians. Thereafter, he supported the Church financially, built various basilicas, granted privileges (e.g., exemption from certain taxes) to clergy, promoted Christians to high ranking offices, and returned property confiscated during the reign of Diocletian.Gerberding, pp.55–56 Constantine utilized Christian symbols early in his reign but still encouraged traditional Roman religious practices including sun worship. Between 324 and 330, he built a new imperial capital at Byzantium on the Bosphorus (it came to be named for him: Constantinople)–the city employed overtly Christian architecture, contained churches within the city walls (unlike "old" Rome), and had no pagan temples.Gerberding, p. 56 In 330 he established Constantinople as the new capital of the Roman Empire. The city would gradually come to be seen as the center of the Christian world.Payton (2007), p.29
Sociologist Joseph Bryant asserts that, by the time of Constantine, Christianity had already changed from its first century instantiation as a "marginal, persecuted, and popularly despised Christian sect" to become the fully institutionalized church "capable of embracing the entire Roman empire" that Constantine adopted. Without this transformation that Peter Brown has called "the conversion of Christianity" to the culture and ideals of the Roman world, Brown says Constantine would never have converted himself.
By the end of the second century Christianity was steadily expanding and its membership was socially rising. The church was becoming increasingly institutionalized, and there is evidence of moral erosion and declining commitment amongst its expanding membership. Bryant explains that, "The governing principle of the [sect is] in the personal holiness of its members". A church, on the other hand, is an organization where sanctity is found in the institution rather than the individual. To become a church, "Christianity had to overcome its alienation from the 'world' and successfully weather persecution, accept that it was no longer an ecclesia pura, (a sect of the holy and the elect), but was instead a corpus permixtum, a 'catholic' Church geared to mass conversions and institutionally endowed with extensive powers of sacramental grace and redemption". This "momentous transformation" threatened the survival of the marginal religious movement as it naturally led to divisions, schisms and defections. Bryant explains that, "once those within a sect determine that "the 'spirit' no longer resides in the parent body, 'the holy and the pure' typically find themselves compelled – either by conviction or coercion – to withdraw and establish their own counter-church, consisting of the 'gathered remnant' of God's elect". According to Bryant, this describes all the schisms of Christianity's first 300 years including the Montanists, the schism created by Hippolytus in 218 under Callistus, the Melitian schism, and the Donatists.
It is the Donatist schism that Bryant sees as the culmination of this sect to church dynamic. During the Melitian schism and the beginnings of the Donatist division, bishop Cyprian had felt compelled to "grant one laxist concession after another in the course of his desperate struggle to preserve the Catholic church". Roman emperors had always been religious leaders, but Constantine established precedent for the position of the Christian emperor in the Church. These emperors considered themselves responsible to God for the spiritual health of their subjects, and thus they had a duty to maintain orthodoxy.Richards, pp.14–15 The emperor did not decide doctrine – that was the responsibility of the bishops – rather his role was to enforce doctrine, root out heresy, and uphold ecclesiastical unity.Richards, p. 15 The emperor ensured that God was properly worshiped in his empire; what proper worship consisted of was the responsibility of the church. Constantine had commissioned more than one investigation into the Donatist issues and they all ruled in support of the Catholic cause, yet the Donatists refused to submit to either imperial or ecclesiastical authority. For a Roman emperor, that was sufficient cause to act. Brown says Roman authorities had shown no hesitation in "taking out" the Christian church they had seen as a threat to empire, and Constantine and his successors did the same, for the same reasons.Peter Brown, The Rise of Christendom 2nd edition (Oxford, Blackwell Publishing, 2003) Constantine's precedent of deferring to councils on doctrine, and accepting responsibility for their enforcement, would continue generally until the empire's end, although there were a few emperors of the 5th and 6th centuries who sought to alter doctrine by imperial edict without recourse to councils.Richards, p.16
In 325 Constantine called for the Council of Nicaea, which was effectively the first Ecumenical Council (the Council of Jerusalem was the first recorded Christian council but rarely is it considered ecumenical), to deal mostly with the Arian heresy, but which also issued the Nicene Creed, which among other things professed a belief in One Holy Catholic Apostolic Church, the start of Christendom. John Kaye characterizes the conversion of Constantine, and the council of Nicea, as two of the most important things to ever happen to the Christian church.KAYE, John. Some account of the Council of Nicæa in connexion with the life of Athanasius. United Kingdom, n.p, 1853.
Constantius II Constantine's sons banned pagan State religious sacrifices in 341 but did not close the temples. Although all State temples in all cities were ordered shut in 356, there is evidence that traditional sacrifices continued. When Gratian declined the position and title of Pontifex Maximus, his act effectively brought an end to the state religion because of the position's authority and ties within the administration. This ended state official practices but not the private religious practices, and consequently the temples remained open.
There was not a total unity of Christianity however, and Constantius II was an Arian who kept Arian bishops at his court and installed them in various sees, expelling the orthodox bishops.
Julian the Apostate Constantius's successor, Julian, known in the Christian world as Julian the Apostate, was a philosopher who upon becoming emperor renounced Christianity and embraced a Neo-platonic and mystical form of paganism shocking the Christian establishment. While not actually outlawing Christianity, he became intent on re-establishing the prestige of the old pagan beliefs and practices. He modified these practices to resemble Christian traditions such as the episcopal structure and public charity (hitherto unknown in Roman paganism). Julian eliminated most of the privileges and prestige previously afforded to the Christian Church. His reforms attempted to create a form of religious heterogeneity by, among other things, reopening pagan temples, accepting Christian bishops previously exiled as heretics, promoting Judaism, and returning Church lands to their original owners. However, Julian's short reign ended when he died while campaigning in the East. Christianity came to dominance during the reign of Julian's successors, Jovian, Valentinian I, and Valens (the last Eastern Arian Christian emperor).
Nicaea Christianity becomes the state religion of the Roman Empire 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. By 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 Diocletian persecutions, created a crisis in the western Empire.Irvin (2002), p. 164, ch. 15. A Church synod, or council, was called in Rome in 313 followed by another in Arles in 314. The latter was presided over by Constantine while he was still a junior emperor. The councils 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. This was only the beginning of imperial involvement in the Christian theology.