Political expediency - The Roman Emperor Constantine won the war and all the new territory he was going to govern had a large Christian population.
The Christians also incorporated some of the Romans idolatry ways of worshiping their many gods into Christianity to make it appealing to the Romans.
Christianity did not easily spread through the Roman Empire. See the related question, "How did Christianity spread throughout the Roman Empire?"
The Roman Empire.
That would be Christianity.
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 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
Through most of their history the ancient Romans had their own religion: Roman Religion. In the Later Empire Christianity spread in the Roman Empire, was endorsed by the Roman emperors and became state religion.
Christianity was certainly a constant. Christianity was also spread beyond the area of the former western part of the Roman Empire with the conversion of previously pagan peoples.
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.