During the reign of the Roman Emperor Constantine the Great (AD 306-337), Christianity began to transition to the dominant religion of the Roman Empire. Historians remain uncertain about Constantine's reasons for favoring Christianity, and theologians and historians have often argued about which form of early Christianity he subscribed to. There is no consensus among scholars as to whether he adopted his mother Helena's Christianity in his youth, or, as claimed by Eusebius of Caesarea, encouraged her to convert to the faith he had adopted himself.
Constantine ruled the Roman Empire as sole emperor for much of his reign. Some scholars allege that his main objective was to gain unanimous approval and submission to his authority from all classes, and therefore chose Christianity to conduct his political propaganda, believing that it was the most appropriate religion that could fit with the Imperial cult (see also Sol Invictus). Regardless, under the Constantinian dynasty Christianity expanded throughout the Empire, launching the era of State church of the Roman Empire. Whether Constantine sincerely converted to Christianity or remained loyal to Paganism is still a matter of debate among historians (see also Constantine's religious policy). His formal conversion in 312 is almost universally acknowledged among historians, despite that he was baptized only on his deathbed by the Arian bishop Eusebius of Nicomedia in 337; the real reasons behind it remain unknown and are debated also. According to Hans Pohlsander, Professor Emeritus of History at the University at Albany, SUNY, Constantine's conversion was just another instrument of realpolitik in his hands meant to serve his political interest in keeping the Empire united under his control:
The prevailing spirit of Constantine's government was one of conservatorism. His conversion to and support of Christianity produced fewer innovations than one might have expected; indeed they served an entirely conservative end, the preservation and continuation of the Empire.-- Hans Pohlsander, The Emperor Constantine
Constantine's decision to cease the persecution of Christians in the Roman Empire was a turning point for early Christianity, sometimes referred to as the Triumph of the Church, the Peace of the Church or the Constantinian shift. In 313, Constantine and Licinius issued the Edict of Milan decriminalizing Christian worship. The emperor became a great patron of the Church and set a precedent for the position of the Christian emperor within the Church and raised the notions of orthodoxy, Christendom, ecumenical councils, and the state church of the Roman Empire declared by edict in 380. He is revered as a saint and isapostolos in the Eastern Orthodox Church, Oriental Orthodox Church, and various Eastern Catholic Churches for his example as a "Christian monarch".
The first recorded official persecution of Christians on behalf of the Roman Empire was in AD 64, when, as reported by the Roman historian Tacitus, Emperor Nero attempted to blame Christians for the Great Fire of Rome. According to Church tradition, it was during the reign of Nero that Peter and Paul were martyred in Rome. However, modern historians debate whether the Roman government distinguished between Christians and Jews prior to Nerva's modification of the Fiscus Judaicus in 96, from which point practicing Jews paid the tax and Christians did not.
Christians suffered from sporadic and localized persecutions over a period of two and a half centuries. Their refusal to participate in the Imperial cult was considered an act of treason and was thus punishable by execution. The most widespread official persecution was carried out by Diocletian. During the Great Persecution (303-311), the emperor ordered Christian buildings and the homes of Christians torn down and their sacred books collected and burned. Christians were arrested, tortured, mutilated, burned, starved, and condemned to gladiatorial contests to amuse spectators. The Great Persecution officially ended in April 311, when Galerius, senior emperor of the Tetrarchy, issued an edict of toleration, which granted Christians the right to practice their religion, though it did not restore any property to them. Constantine, Caesar in the Western empire, and Licinius, Caesar in the East, also were signatories to the edict of toleration. It has been speculated that Galerius' reversal of his long-standing policy of Christian persecution has been attributable to one or both of these co-Caesars.
It is possible (but not certain) that Constantine's mother, Helena, exposed him to Christianity; in any case he only declared himself a Christian after issuing the Edict of Milan. Writing to Christians, Constantine made clear that he believed that he owed his successes to the protection of that High God alone.
Eusebius of Caesarea and other Christian sources record that Constantine experienced a dramatic event in 312 at the Battle of the 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 " ?" (in this sign, conquer), often rendered in a Latin version, "in hoc signo vinces" (in this sign, you will conquer). Constantine commanded his troops to adorn their shields with a Christian symbol (the Chi-Rho), and thereafter they were victorious.
Following the battle, the new emperor ignored the altars to the gods prepared on the Capitoline and did not carry out the customary sacrifices to celebrate a general's victorious entry into Rome, instead heading directly to the imperial palace. Most influential people in the empire, however, especially high military officials, had not been converted to Christianity and still participated in the traditional religions of Rome; Constantine's rule exhibited at least a willingness to appease these factions. The Roman coins minted up to eight years after the battle still bore the images of Roman gods. The monuments he first commissioned, such as the Arch of Constantine, contained no reference to Christianity.
In 313 Constantine and Licinius announced "that it was proper that the Christians and all others should have liberty to follow that mode of religion which to each of them appeared best," thereby granting tolerance to all religions, including Christianity. The Edict of Milan went a step further than the earlier Edict of Toleration by Galerius in 311, returning confiscated Church property. This edict made the empire officially neutral with regard to religious worship; it neither made the traditional religions illegal nor made Christianity the state religion, as occurred later with the Edict of Thessalonica of 380. The Edict of Milan did, however, raise the stock of Christianity within the empire and it reaffirmed the importance of religious worship to the welfare of the state.
The accession of Constantine was a turning point for early Christianity. After his victory, Constantine took over the role of patron of the Christian faith. He supported the Church financially, had an extraordinary number of basilicas built, granted privileges (e.g., exemption from certain taxes) to clergy, promoted Christians to high-ranking offices, returned property confiscated during the Great Persecution of Diocletian, and endowed the church with land and other wealth. Between 324 and 330, Constantine built a new imperial capital at Byzantium on the Bosporos, which would be named Constantinople for him. Unlike "old" Rome, the city began to employ overtly Christian architecture, contained churches within the city walls, and had no pre-existing temples from other religions.
In doing this, however, Constantine required those who had not converted to Christianity to pay for the new city. Christian chroniclers tell that it appeared necessary to Constantine "to teach his subjects to give up their rites ... and to accustom them to despise their temples and the images contained therein," This led to the closure of temples because of a lack of support, their wealth flowing to the imperial treasure; Constantine did not need to use force to implement this. Only the chronicler Theophanes has added that temples "were annihilated", but this was considered "not true" by contemporary historians.
Constantine respected cultivated persons, and his court was composed of older, respected, and honored men. Men from leading Roman families who declined to convert to Christianity were denied positions of power yet still received appointments; even up to the end of his life, two-thirds of his top government were non-Christian.
Constantine's laws enforced and reflected his Christian attitudes. crucifixion was abolished for reasons of Christian piety but was replaced with hanging, to demonstrate the preservation of Roman supremacy. On March 7, 321, Sunday, already sacred to Christians and to the Roman Sun God Sol Invictus, was declared an official day of rest. On that day markets were banned and public offices were closed, except for the purpose of freeing slaves. There were, however, no restrictions on performing farming work, which was the work of the great majority of the population, on Sundays.
Some laws made during his reign were even humane in the modern sense, possibly inspired by his Christianity: a prisoner was no longer to be kept in total darkness but must be given the outdoors and daylight; a condemned man was allowed to die in the arena, but he could not be branded on his "heavenly beautified" face, since God was supposed to have made man in his image, but only on the feet. Publicly displayed gladiatorial games were ordered to be eliminated in 325.
In 331, Constantine commissioned Eusebius to deliver fifty Bibles for the Church of Constantinople. Athanasius (Apol. Const. 4) recorded around 340 Alexandrian scribes preparing Bibles for Constans. Little else is known. It has been speculated that this may have provided motivation for canon lists, and that Codex Vaticanus and Codex Sinaiticus are examples of these Bibles. Together with the Peshitta and Codex Alexandrinus, these are the earliest extant Christian Bibles.
The reign of Constantine established a precedent for the position of the Christian emperor in the Church. Emperors considered themselves responsible to the gods for the spiritual health of their subjects, and after Constantine they had a duty to help the Church define orthodoxy and maintain orthodoxy. The Church generally regarded the definition of doctrine as the responsibility of the bishops; the emperor's role was to enforce doctrine, root out heresy, and uphold ecclesiastical unity. The emperor ensured that God was properly worshiped in his empire; what proper worship (orthodoxy) and doctrines and dogma consisted of was for the Church to determine.
Constantine had become a worshiper of the Christian God, but he found that there were many opinions on that worship and indeed on who and what that God was. In 316, Constantine was asked to adjudicate in a North African dispute of the Donatist sect (who began by refusing obedience to any bishops who had yielded in any way to persecution, later regarding all bishops but their own sect as utterly contaminated). More significantly, in 325 he summoned the First Council of Nicaea, effectively the first Ecumenical Council (unless the Council of Jerusalem is so classified). The Council of Nicaea is the first major attempt by Christians to define orthodoxy for the whole Church. Until Nicaea, all previous Church Councils had been local or regional synods affecting only portions of the Church.
Nicaea dealt primarily with the Arian controversy. Constantine himself was torn between the Arian and Trinitarian camps. After the Nicene council, and against its conclusions, he eventually recalled Arius from exile and banished Athanasius of Alexandria to Trier.
Just before his death in May 337, Constantine was baptised into Christianity. Up until this time he had been a catechumen for most of his adult life. He believed that if he waited to get baptized on his death bed he was in less danger of polluting his soul with sin and not getting to heaven. He was baptized by his distant relative Arian Bishop Eusebius of Nicomedia. During Eusebius of Nicomedia's time in the Imperial court, the Eastern court and the major positions in the Eastern Church were held by Arians or Arian sympathizers. With the exception of a short period of eclipse, Eusebius enjoyed the complete confidence both of Constantine and Constantius II and was the tutor of Emperor Julian the Apostate. After Constantine's death, his son and successor Constantius II was an Arian, as was Emperor Valens.
Constantine's position on the religions traditionally practiced in Rome evolved during his reign. In fact, his coinage and other official motifs, until 325, had affiliated him with the pagan cult of Sol Invictus. At first, Constantine encouraged the construction of new temples and tolerated traditional sacrifices; by the end of his reign, he had begun to order the pillaging and tearing down of Roman temples.
Beyond the limes, east of the Euphrates, the Sasanian rulers of the Persian Empire, perennially at war with Rome, had usually tolerated Christianity. Constantine is said to have written to Shapur II in 324 and urged him to protect Christians under his rule. With the establishment of Christianity as the state religion of the Roman Empire, Christians in Persia would be regarded as allies of Persia's ancient enemy. According to an anonymous Christian account, Shapur II wrote to his generals:
You will arrest Simon, chief of the Christians. You will keep him until he signs this document and consents to collect for us a double tax and double tribute from the Christians ... for we Gods  have all the trials of war and they have nothing but repose and pleasure. They inhabit our territory and agree with Caesar, our enemy.-- Shapur II, A History of Christianity in Asia: Beginnings to 1500
Constantinian shift is a term used by some theologians and historians of antiquity to describe the political and theological aspects and outcomes of the 4th-century process of Constantine's integration of the Imperial government with the Church that began with the First Council of Nicaea. The term was popularized by the Mennonite theologian John H. Yoder.
The claim that there ever was Constantinian shift has been disputed; Peter Leithart argues that there was a "brief, ambiguous 'Constantinian moment' in the fourth century," but that there was "no permanent, epochal 'Constantinian shift'."
Constantine was not a young convert. He was over 40 and an experienced politician when he finally declared himself a Christian. He had had time to take the measure of the new religion and the difficulties which emperors had experienced in suppressing it. He decided that Christianity was a religion fit for a new empire.
What might be called the Constantinian shift began around the year 200 and took more than two hundred years to grow and unfold to full bloom.
The most impressive transitory change underlying our common experience, one that some thought was a permanent lunge forward in salvation history, was the so-called Constantinian shift.