At first Christianity was persecuted. Initially it was used as a scapegoat, later it was seen as a threat to the Roman state because Roman religious practices were seen as part of the foundation of the state.
The persecutions were stopped and Christianity became tolerated because the last persecution had led to widespread unrest as the majority of people had become Christians. This restored political stability and gave the empire a greater sense of unity.
Conflicts between Christians also developed. Mainstream Christianity, Latin (western) and Greek (eastern) Christianity (later they came to be called Catholic and Orthodox) shared the Nicaean interpretation of the trinity and were opposed to alternative interpretations and other sects of Christianity. Mainstream Christianity was made state religion and the other Christian sects were banned. Tis led to conflict between mainstream Christianity and the biggest of the banned sects Arian Christianity, which was popular around the empire.
Christianity became part of the identity of the empire.
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.