Roman emperor Theodosius I proclaims his eight-year-old son Honorius co-emperor.
Theodosius I, often called *Theodosius the Great*, was Roman emperor from 379 to 395 CE and is best known as the last ruler to govern both the Eastern and Western Roman Empires together. Born in Hispania around 347 CE, he rose to prominence as a military commander before being appointed emperor of the Eastern Roman Empire by Gratian after the disastrous Battle of Adrianople, where the Romans suffered a crushing defeat by the Goths. Theodosius inherited an empire under severe pressure from internal instability and external invasions, and much of his early reign was spent restoring military control and negotiating with barbarian groups, notably settling the Goths within imperial borders as federated allies.

One of Theodosius I’s most significant and lasting impacts was his religious policy. A committed Christian, he strongly supported Nicene (orthodox) Christianity and worked actively to suppress paganism and heretical forms of Christianity. In 380 CE, he issued the Edict of Thessalonica, which declared Nicene Christianity the official state religion of the Roman Empire. Over the following years, he closed or restricted pagan temples, ended state support for traditional Roman religious practices, and reinforced the authority of the Christian Church. His reign marked a decisive turning point in the Christianization of the Roman state, reshaping its cultural and religious identity for centuries to come.

Politically and militarily, Theodosius managed to reunite the empire in 394 CE after defeating the usurper Eugenius in the Battle of the Frigidus, briefly ruling as sole emperor. However, his reign also exposed the empire’s growing dependence on non-Roman troops and fragile internal cohesion. Upon his death in 395 CE, Theodosius divided the empire permanently between his two sons—Arcadius in the East and Honorius in the West. This division proved irreversible, and while the Eastern Empire would endure for another thousand years, the Western Empire soon declined. As a result, Theodosius I is remembered both as a unifier and as a transitional figure at the end of the classical Roman world.