Edict of Toleration 311 and Edict of Milan
When the Roman Emperor Diocletian abdicated, Galerius became senior emperor in 305. He continued the cruel persecution, of Christians which was so widespread and intense that it became known as the great persecution. However, Christianity simply did not decline. Even Galerius recognized the impossibility of eradicating the illegal religion.
However, when the emperor became seriously ill, he connected his persecution of Christians as responsible for his severe illness. He seems to have believed his illness was as a judgment from the Christian God. Shortly before his death, The Edict of Toleration on April 30, 311 was issued in Serdica (Sofia, Bulgaria) by Galerius, officially ending Diocletian’s persecution of Christianity..
In that document he declared, that the purpose of reclaiming the Christians from their willful innovation and the multitude of their sects to the laws and discipline of the, Roman state, was not accomplished; and that he would now grant them permission to hold their religious assemblies provided they disturbed not the order of the state. To this he added in conclusion the significant instruction that the Christians, “after this manifestation of grace, should pray to their God for the welfare of the emperors, of the state, and of themselves, that the state might prosper in every respect, and that they might live quietly in their homes.
His successor, Emperor Maximinus, tried to counteract the edict but did not succeed to any great extent in his short rule. The Great Persecution of Christians had officially ended.
Edict of Milan
Two years later, the Western Roman Co-regent Constantine , and his brother-in-law, Co-regent Licinius, who controlled the Balkans, met in Milan and agreed to change policies towards Christians. The second edict went beyond the first of 311; it was a decisive step from hostile neutrality to friendly neutrality and protection, and prepared the way for the legal recognition of Christianity, as the religion of the empire. It ordered the full restoration of all confiscated church property to the Corpus Christianorum, at the expense of the imperial treasury, and directed the provincial magistrates to execute this order at once with all energy, so that peace may be fully established and the continuance of the Divine favor secured to the emperors and their subjects.
Constantine issued the Edict of Milan in 313, thus ending all Roman-sponsored persecution of Christianity, the Empire became a haven for Christians. Constantine I is believed to have converted himself to Christianity just before a decisive battle in 312. The agreement required that Christians be treated benevolently within the Roman Empire. Maximin, also, shortly before his suicide (313), was compelled to give his consent at to the Edict.
The Edict implicitly granted Christianity the status of “religio licita”, a worship recognized and accepted by the Roman Empire. It was the first edict legalizing Christianity. The Edict of Milan gave Christianity a legal status, but did not make Christianity the official religion of the Roman empire.
Just twelve years later in 325, Constantine assembled the First Council of Nicaea. On July 4, 325, about three hundred Christian bishops and deacons from the eastern half of the Roman Empire had come to Nicea, a small town near the Bosporus Straits. In the conference hall where they waited for the Emperor was an open copy of the Gospels. After three centuries of persecutions instigated by Roman emperors, the bishops and deacons were gathered not as enemies but as allies with the Emperor.
Nicea symbolized a new day for Christianity. The persecuted followers of the Savior dressed in linen had become the respected advisers of emperors robed in purple. The once-despised religion was on its way to becoming the state religion, the spiritual cement of a single society in which public and private life were united under the control of Christian doctrine.
Constantine, met with around 220 Christian leaders, but out of respect, without his customary train of soldiers. Constantine spoke briefly. He told the churchmen they had to come to an agreement on the crucial questions dividing them. “Division in the church,” he said, “is worse than war.” The problem that Constantine expected the bishops to solve was the dispute over Arianism.
Arius, pastor of the influential Baucalis Church in Alexandria, Egypt, taught that Christ was more than human but something less than God. He said that God originally lived alone and had no Son. Then he created the Son, who in turn created everything else. The idea persists in some cults today. Arius’s teaching held a special appeal for many recent converts to Christianity. It was like the pagan religions of their childhood: the one supreme God, who dwells alone, makes a number of lesser gods who do God’s work, passing back and forth from heaven to earth. These former pagans found it hard to understand the Christian belief that Christ, the Divine Word, existed from all eternity, and that he is equal to the Almighty Father. So Arianism spread, creating Constantine’s concern. 
The bishops, met for two months to hammer out a universally acceptable definition of Jesus Christ. Once the Council of Nicea convened, most of the bishops were ready to compromise. Most of the pastors, however, recognized that more was needed to exclude Arian teaching. For this purpose they produced another creed, They inserted an extremely important series of phrases: “True God of true God, begotten not made, of one substance with the Father….”
After an extended debate, all but two bishops at the council agreed upon a creed that confessed faith “in one Lord Jesus Christ, … true God of true God.” Constantine was pleased, and believed the issue was settled. However, Nicea alone settled little. For the next century the Nicene and the Arian views of Christ battled for supremacy. First Constantine and succeeding Emperors continued to banish and exile churchmen. Control of church offices too often depended on control of the emperor’s favor.
Paganism made another effort to challenge Christianity in the first half of the third century. Co-regent Licinius fell out with Emperor Constantine and renewed the persecution of Christians for a short time in the East, but he was defeated in 323, and Constantine became sole ruler of the empire (Emperor Constantine I the Great). He openly protected and favored the church, without forbidding idolatry, and upon the whole remained true to his policy of protective toleration till his death (337). This was enough for the success of the church, which had all the vitality and energy of a victorious power; while heathenism was fast decaying at its root.
In 324 AD from ancient Byzantium as the new capital of the Roman Empire by Emperor Constantine the Great, after whom it was named, and dedicated on 11 May 330 AD.[ . As the seat of the Roman Empire and Christianity the city would be instrumental in the advancement of Christianity during Roman and Byzantine times as the home of the Ecumenical Patriarch of Constantinople and as the guardian of Christendom’s holiest relics such as the Crown of Thorns and the True Cross.
The following from the time of Emperor Theodosius the Great (383–395) enforced the Christian religion to the exclusion of every other; and not only so, but they enforced orthodoxy to the exclusion of every form of dissent, which was punished as a crime against the state.
In 391, EmperorTheodosius I made Christianity the official religion of Rome. By the 4th century, the Thracians, Greeks and other peoples of the southern Balkans had a composite indigenous identity, as Christian “Romans” who preserved some of their ancient pagan rituals.
 Edward Gibbon (1 January 2008). The History of the Decline and Fall of the Roman Empire