Trajan Decius was the Roman emperor from 249-251 CE, and the first of many emperors for more than half a century to persecute Jews and Christians as official policy. He required members of minority religions to make public sacrifices, offerings, and statements of faith to Roman gods, on pain of death. Some Christians were particularly eager to be martyred for their faith, often seeking out Roman inquisitors to prove their devotion to Christ even in the face of imprisonment and torture.
In 303 Emperors Diocletian, Maximian, Galerius, and Constantius issued a series of edicts restricting the religious rights of Jews and Christians. This era is known in the Christian faith as the Great Persecution, an isolated era of history rather than its theme, and many sainted martyrs are venerated for resisting Roman persecution. The Roman empire at that time stretched across the Mediterranean and up into Britain, which was not yet an independent country but a farflung Roman trading outpost state, where enforcement of religious laws was minimal and sporadic.
Emperors Constantine I, also known as Constantine the Great, and Licinius signed the Edict of Milan in 313. This legalized Christianity across the whole of the Roman empire, stretching from modern day Spain through north Africa across the Mediterranean to the Black Sea. After a period of shared power among emperors, Constantine defeated his fellows to become the sole emperor of Rome in 324. When he did so, he made Christianity the official state religion.
Constantine’s mother Helena was Christian and, more politically important, several important bishops were. Constantine was said to have had a vision of the Christian cross (or the sun god Sol Invictus) before his military success at the Battle of Milvian Bridge in 317. But from the signing of the Edict of Milan, the fortunes of Christians in the Roman empire improved, and Jewish persecution increased once again.
In 324 as sole emperor, Constantine I moved the capital of the empire from Rome to Byzantine and changed the name of that city to Constantinople, after himself. Today it is called Istanbul and is the capital city of Turkey. His empire was now larger than ever, entailing the Latin and Greek speaking Christian world of the early 4th century. Constantine called bishops from every state of his vast empire to the First Council of Nicea (in what is now Iznik, in the Bursa province of Turkey). This grand council was intended to settle theological disputes in Christendom and create a unified theology and organized hierarchy of clergy.
A Noble Death: Suicide and Martyrdom Among Christians and Jews in Antiquity
Eusebius, Life of Constantine, Book III