Theodosius (I) the Great,(346?–;395), a Roman emperor, the last to rule over a unified empire. He is called “Great” because he made Catholic Christianity the only legal religion of the Roman Empire. His action brought to an end the open practice of paganism in most of Europe and the Middle East.

Theodosius, the son of a Roman general, was born in Spain. He chose a military career and distinguished himself in service against the barbarians pressing across the frontiers. In 378 Valens, Roman emperor of the East, was slain at the Battle of Adrianople by the Visigoths. The following year Gratian, Roman emperor of the West, selected Theodosius to succeed Valens and drive back the Visigoths. Theodosius expelled them from the lower Balkan Peninsula, but was unable to push the main body of the Visigoths back across the Danube. He then signed a peace treaty with the Visigoths in 382 that made them responsible for defending the empire's frontiers.

To resolve the conflict between the Nicene and Arian branches of Christianity, in 381 Theodosius called a church council at Constantinople (later called the Second Ecumenical Council), which reaffirmed the Nicene Creed. Theodosius imposed the Nicene interpretation of the Christian faith on the entire empire. He punished Arians as heretics and destroyed pagan temples. He also destroyed the Alexandrian Library, which was considered a center of pagan learning. The Olympic Games were abolished because they were a pagan festival.

Twice during his reign Theodosius was called to Italy to remove illegal rulers. In 394 he assumed control of the entire empire. On his deathbed the following year he bequeathed the eastern part of the empire to his son Arcadius and the western part to his son Honorius. This division of the empire was permanent.