Christianity was made the official religion of the Roman Empire by Emperor Theodosius I. In 380 CE, Theodosius issued the Edict of Thessalonica, which established Nicene Christianity as the state religion and made adherence to it compulsory, marking the formal declaration of Christianity as the official faith of the empire. This edict also branded other religious practices and Christian sects as heretical, beginning a new era of religious uniformity and the marginalization of paganism and other beliefs within the empire.
Before Theodosius, Emperor Constantine the Great had played a critical role in promoting Christianity by ending persecution and supporting its spread, but it was Theodosius who legally established it as the empire's official religion.
