Byzantine Empire
During the high period of the Roman Empire known as the Pax Romana, the western parts of the empire went through Latinization, while the eastern parts of the empire maintained to a large degree their Helenistic culture. Several events from the 4th to 6th centuries mark the period of transition during which the Roman Empire's  Greek East and Latin West diverged. Constantine I (r. 324–337) reorganised the empire, made Constantinople the capital, and legalised Christianity. Under Theodosius I (r. 379–395), Christianity became the state relegion, and other religious practices were proscribed. In the reign of Heraclius (r. 610–641), the empire's military and administration were restructured and Greek was gradually adopted for official use in place of Latin.