Since a persecuted church is a pure church, some would point to the cessation of persecution around the time of Constantine as a backward step, as the church lost this purifying influence. Simultaneously, Constantine showed favor to the Christians and so it became to a degree fashionable to be Christian. This brought into the Christian church many who were Christian in name only and thus it ceased to be true to itself. Around this time there was an increased prosperity for the Christians, partly due to state patronage and so the trust of some tended to move away from the master to mammon.
All of the above factors tended to reduce the uniqueness of the Christian message and certainly to grieve the Spirit of God and thus diminish the power of the word in terms of true life-changing conversion.
Christianity is a proselytising religion, and many missionaries are spread around the word preaching to the unconverted. Two factors that could inhibit the further spread of Christianity are:
Faith. Many people sincerely follow other religions and see these as more true than Christianity. So they may see no real reason to become Christians.
Education. Studies have shown that the more educated a person is, the less likely he or she is to believe in religion.
During the Pax Romana, he promoted Christianity even though he was a Pagan.
Christianity is the contribution of the Romans to religion. Christianity spread around the Roman Empire and eventually became state religion. Both Catholic and Orthodox Christianity originally developed in the Roman Empire.
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It continued the Holy Roman empire in the East, and spread Christianity to Russia. Also great buildings like the Hagia Sophia
The Romans did not try to stop the spread of Christianity. There were alternations between periods of toleration and periods of persecution. Some Roman emperors thought that Christianity was a threat to the Roman state because many bureaucrats and soldiers converted to Christianity. The Romans saw their state as being interlinked with Roman Religion. The persecutions were aimed mainly at the civil service and the Christian clergy, rather than Christianity as such. The last and worse persecution, the Great Persecution, failed because it was difficult to implement, because of the resistance of the Christians and because it undermined the stability of the empire. After that, the emperors endorsed Christianity, favored it over other religions and promoted it. Eventually, Christianity became the state religion of the empire. With imperial support, the spread of Christianity accelerated.
Christianity did not easily spread through the Roman Empire. See the related question, "How did Christianity spread throughout the Roman Empire?"
The spread of Christianity had political ramifications for the Roman Empire. The empire split into two, Constantinople to the east and Rome to the west
Christianity
The Roman Empire made Christianity the official religion of the Empire, and 'encouraged' the peoples it conquered to convert.
The Roman Empire.
The Romans who remained pagans did not contribute to the spread of Christianity Neither did Julian, who was the only pagan emperor in the Later Roman Empire
1018
As Roman Emporer, Constantine declared Christianity to be the official religion of the Roman Empire. At the time, the Empire was vast, so Christianity spread quickly.
By 325ad Christianity had spread throughout the Roman Empire and to Britain. If you google "spread of Christianity map" you will find maps showing the spread.
Christianity developed from a religion among a small group of Jews (who lived in Judea, which was part of the Roman Empire) into a mass religion in the Roman days. It spread around the Roman Empire. It became state religion. Catholic Christianity and Orthodox Christianity developed during the Later Roman Empire. They were originally called Latin or Western Christianity and Greek or Eastern Christianity respectively. The former was the main form of Christianity in the western part of the Roman Empire and the latter was the main form of Christianity in the eastern part of the Roman Empire.
There was only one Roman Empire. Moreover, the Roman Empire did not study Christianity. An empire cannot study. The Roman Empire was where Christianity spread from Judea (which was a part of the Roman province of Syria). Both Catholic and Orthodox Christianity developed in the Roman Empire. Originally the were called Latin or Western Christianity and Greek or Eastern Christianity respectively. The former was the dominant form of Christianity in the western part of the Roman Empire and the latter was the dominant form of Christianity in the eastern part of this empire. There were also dissident Christian doctrines and sects.
During the Pax Romana, he promoted Christianity even though he was a Pagan.