Constantine the Great had himself baptised just before his death.
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.
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.
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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
Which Roman leader exactly? Whom are your referring to?
Christianity did not easily spread through the Roman Empire. See the related question, "How did Christianity spread throughout the Roman Empire?"
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
They used force
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
It could not decline from nothing - it did not exist until after much of what was to become the Roman empire was already under Roman rule. It spread, particularly after the Romans accepted Christianity under Constantine
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.
The Roman Empire made Christianity the official religion of the Empire, and 'encouraged' the peoples it conquered to convert.
The pastor. ---- The Pope, With most of the Christians in the world being Roman Catholics, The Roman Catholic Church is the face of Christianity to the non-Christian world, as such, the Pope is the most major religious leader in Christianity.