Constantine the Great and Christianity
During the reign of the Roman emperor Constantine the Great, 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.
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 he chose Christianity to conduct his political propaganda, believing that it was the most appropriate religion that could fit with the imperial cult. Regardless, under the Constantinian dynasty Christianity expanded throughout the empire, launching the era of the state church of the Roman Empire. Whether Constantine sincerely converted to Christianity or remained loyal to paganism is a matter of debate among historians. His formal conversion in 312 is almost universally acknowledged among historians, despite that it was claimed 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.
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.
Before Constantine
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 practising 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 beginning in 303. During the Great Persecution, 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, although 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. 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.
Constantine's conversion
It is possible 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 the High God alone.Vision of Apollo
In 310 a panegyric, preserved in the Panegyrici Latini collection and delivered at Trier for the joint occasion of the city's birthday and Constantine's quinquennalia, recounted a vision apparently seen by the emperor while journeying between Marseille and Trier. The panegyricist recounts that the god Apollo appeared to Constantine in company with Victoria and together presented him with three wreaths representing thirty years of power. This vision was perhaps in a dream experienced by the emperor while practising incubation at the shrine of Apollo Grannus in Grand, Vosges. Eusebius was aware of this vision, or reports of it, and refers in his own Panegyric of Constantine of 336 to "tricennial crowns" bestowed by the hand of God in Christianity on Constantine, "augmenting the sway of his kingdom by long years".Battle of Milvian Bridge
Eusebius of Caesarea and other Christian sources record that Constantine experienced a dramatic series of events sometime between his father Constantius Chlorus's death in 306 and the Battle of the Milvian Bridge on 28 October 312. The battle secured Constantine's claim to the title of augustus in the West, which he had assumed unilaterally when his father died. According to the Eusebius' Life of Constantine, Constantine saw a vision of "a cross-shaped trophy formed from light" above the sun at midday.The Greek words Ἐν Τούτῳ Νίκα 'in this sign, conquer' are often rendered in Latin as in hoc signo vinces 'in this sign, you will conquer'. According to Eusebius, Constantine also had a dream that same night. In the dream,
Writing his Church History shortly after 313, Eusebius makes no mention of this story in that work and does not recount it until composing his posthumous biography of Constantine decades afterwards. Life of Constantine was written by Eusebius after Constantine had died, and Eusebius admitted that he had heard the story from Constantine long after it had happened. Lactantius, writing 313–315 and around twenty years before Eusebius's Life, also does not mention a vision in the sky. Instead, Lactantius mentions only that Constantine's dream took place on the eve of the climactic battle on the Ponte Milvio across the Tiber, with the crucial detail that the "sign" was marked on the Constantinian soldiers' shields. According to Lactantius:
File:Konstantin mit Christogramm.jpeg|thumb|Medallion issued by Constantine at Ticinum in 315, with chi-rho on the emperor's crest and Romulus and Remus and the Lupa on his shield.
File:As-Constantine-XR RIC vII 019.jpg|thumb|Follis issued by Constantine at Constantinople in 337, with a chi-''rho on a labarum.
It is unclear from these sources what Constantine saw and what was marked on his army's shields. Eusebius's description of the daytime vision suggests a cross-shaped symbol, whereas Lactantius's description suggests a staurogram, although the crux ansata or the Egyptian ankh have been proposed as interpretations as well. All of these symbols were used by Christians in the 3rd and 4th centuries. Eusebius concurs with Lactantius that a new device was added to Constantine's soldiers' shields but does not connect this with the Battle of the Milvian Bridge, saying only that the "sign of the saving trophy" was marked, but not specifying when. Sometime after 317, Eusebius was permitted by Constantine, probably either in 325 or in 335, to see a standard that was made according to the emperor's dreamt instructions during the civil war. He described it as:
This later description of Eusebius's, written after 324, suggests a more elaborate symbol than does Lactantius's earlier text, involving the Greek letters rho and chi ligature
Following the battle and the defeat and death of Maxentius, Constantine became the undisputed emperor in the West and performed an adventus, a ceremonial entrance to the city. Arriving inside Rome's walls he ignored the altars to the gods prepared on the Capitoline Hill and did not carry out the customary sacrifices to celebrate a general's victorious entry into Rome, instead heading directly to the imperial palace. This is probably because the traditional Roman triumph, concluding with the sacrifice to Jupiter Optimus Maximus'' at his temple on the Capitoline, was traditionally celebrated after victory over Rome's enemies, rather than after the conquest of the city by a claimant in a civil war. The Arch of Constantine, for which numerous reliefs from earlier monuments depicting prior emperors sacrificing to various gods were re-carved with the face of Constantine, does not have an image of Constantine sacrificing to Jupiter, although he is shown sacrificing to Apollo and to Hercules.