Constantine I (27 February 272 – 22 May 337), also known as Constantine the Great, or known mononymously as Constantine, was Roman emperor from AD 306 to 337 and the first Roman emperor to convert to Christianity. He played a pivotal role in elevating the status of Christianity in Rome, the Edict of Milan decriminalising Christian practice and ceasing Christian persecution. This was a turning point in the Christianisation of the Roman Empire. He founded the city of Constantinople (now Istanbul) and made it the capital of the Empire, which it remained for over a millennium.
Born in Naissus, a city located in the province of Moesia Superior (now Niš, Serbia), Constantine was the son of Flavius Constantius, a Roman army officer from Moesia Superior, who would become one of the four emperors of the Tetrarchy. His mother, Helena, was a Greek woman of low birth, probably from Bithynia. Later canonised as a saint, she is credited for the conversion of her son in some traditions, though others believe that Constantine converted her. He served with distinction under emperors Diocletian and Galerius.