Constantine I^[g] (Latin: Flavius Valerius Constantinus; 27 February c. 272 – 22 May 337), also known as Constantine the Great, was a Roman emperor from AD 306 to 337 and the first Roman emperor to convert to Christianity.^[h] He played a pivotal role in elevating the status of Christianity in Rome, decriminalizing Christian practice and ceasing Christian persecution in a period referred to as the Constantinian shift.^[4] This initiated the Christianization of the Roman Empire. Constantine is associated with the religiopolitical ideology known as Caesaropapism, which epitomizes the unity of church and state. He founded the city of Constantinople and made it the capital of the Empire, which remained so for over a millennium.

Born in Naissus, in Dardania within Moesia Superior (now Niš, Serbia), Constantine was the son of Flavius Constantius, a Roman army officer of Illyrian origin who had been one of the four rulers of the Tetrarchy. His mother, Helena, was a Greek woman of low birth, probably from Asia Minor in modern Turkey. Later canonised as a saint, she is traditionally credited for the conversion of her son. Constantine began his career under emperors Diocletian and Galerius in the eastern provinces. He fought the Persians before being recalled west in 305 to campaign alongside his father in the province of Britannia. After his father's death in 306, Constantine was proclaimed as augustus (emperor) by his army at Eboracum (York, England). He eventually emerged victorious in the civil wars against emperors Maxentius and Licinius to become the sole ruler of the Roman Empire by 324.

Upon his ascension, Constantine enacted numerous reforms to strengthen the empire. He restructured the government, separating civil and military authorities. To combat inflation, he introduced the solidus, a new gold coin that became the standard for Byzantine and European currencies for more than a thousand years. The Roman army was reorganised to consist of mobile units (comitatenses), often around the Emperor, to serve on campaigns against external enemies or Roman rebels, and frontier-garrison troops (limitanei) which were capable of countering barbarian raids, but less and less capable, over time, of countering full-scale barbarian invasions. Constantine pursued successful campaigns against the tribes on the Roman frontiers—such as the Franks, the Alemanni, the Goths, and the Sarmatians—and resettled territories abandoned by his predecessors during the Crisis of the Third Century with citizens of Roman culture.