Lucius Caecilius Firmianus signo Lactantius was an early Christian author who became an advisor to Roman emperor Constantine I, guiding his Christian religious policy in its initial stages of emergence, and a tutor to his son Crispus. His most important work is the Institutiones Divinae, an apologetic treatise intended to establish the reasonableness and truth of Christianity to pagan critics.

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.