Roman emperor (288-337 AD). Under his rule, the Christian faith became the official religion of the Roman Empire.
Constantine became Roman emperor in 306 AD, following the death of his father, Constantius Chlorus, in York, England. He took power at a time of great internal turmoil and found a decaying empire from which even some parts of Italy wanted to disengage. He commanded many battles against his rivals that culminated in the defeat of Licinius in Crisopolis and Adrinople in 323 AD.
Constantine played a major role in early Christianity. This is why, from 323 AD, the Christian faith was accepted and even encouraged by the Romans. But it was not so at the beginning of Christianity. In fact, over a period of time, the Christian faith was even tolerated by the Romans. But over time, it began to expand very rapidly and came to be seen as a dangerous threat. Therefore, the emperors began to persecute the Christians. At this time it was common for the pitiful spectacle of Christians to be thrown to the lions at the Coliseum in Rome for amusement of the crowds.
The situation began to change with Constantine. During the battle at Adrinople he would have contemplated a cross in a vision and this led him to credit his victory to Jesus Christ.
Constantine even mediated a major internal dispute over the doctrine between eastern and western factions of the church. In 323 AD, he invited the bishops representing the two groups to a conference in the city of Nicaea, today Iznik, Turkey, where differences were resolved. The Council of Nicaea, outlined at this meeting, defined the basic Christian beliefs that both sides should agree on. Constantine then established Christianity as the official religion of the entire Roman Empire and also took steps to prevent the Christian faith from being destroyed by external persecution or internal conflict. Constantine not only preserved Christianity but also took a major step towards making it the dominant religion in Europe.