The Edict of Milan was the February 313 AD agreement to treat Christians benevolently within the Roman Empire. Western Roman Emperor Constantine I and Emperor Licinius, who controlled the Balkans, met in Mediolanum and, among other things, agreed to change policies towards Christians following the edict of toleration issued by Emperor Galerius two years earlier in Serdica. The Edict of Milan gave Christianity legal status and a reprieve from persecution but did not make it the state church of the Roman Empire, which occurred in AD 380 with the Edict of Thessalonica.

Constantine I, also known as Constantine the Great, was a 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, decriminalizing Christian practice and ceasing Christian persecution in a period referred to as the Constantinian shift. This initiated the cessation of the established ancient Roman religion. Constantine is also the originator of the religiopolitical ideology known as Constantinism, which epitomizes the unity of church and state, as opposed to separation of church and state. He founded the city of Constantinople and made it the capital of the Empire, which remained so for over a millenium.