Flavius Theodosius was born in Spain around 346, the son of the general Theosodius and Thermaitia. He had risen to a high office, but in 375 his father was executed, possibly for a military disgrace. At this point Theodosius retired to his estate in Spain. Whether this retirement was voluntary or not is unclear. He started military service again some time the following year, and following the death of the emperor Valens he was chosen by he emperor Gratian to be the Augustus of the east in January of 379.

He was an active military leader, with successes against the Goths, Saracens, Vandals, and Huns. Much of his effort was also spent in repressing any beliefs other than Christianity. Arianism, Manichaeanism, and other "heresies" were outlawed, as well as the older "pagan" religions. Before his arrival Constantinople had a significant Arian population, including some of it's leaders. Theodosius expelled these, including the bishop Demophilus. Bands of Christian followers and even officials had long been forming mobs to destroy pagan temples and murder the priests. In 391 Theodosius put his official sanction to this, when he ordered the destruction of the Serapium in Alexandria.

In 383 the senior emperor, Gratian, was murdered, and Magnus Maximus usurped the throne of the west. Italy was still held by Valentinian II until 387 when he was expelled by Maximus. Theodosius defeated and executed Maximus, and reinstalled Valentinian. After the death of Valentinian a puppet emperor was set up, but Theodosius marched again on this usurper and was victorius. For the last time, the full Roman Empire was ruled by one man.

Though a staunch supported or the Catholic church, he was actually excommunicated by the bishop of Constantinople in 390, for the massacre of 7000 residents of Thessalonika. A group had risen against the military leader of the garrison there and murdered him, and the massacre was Theodosius' retaliation. He later performed public pennance for this crime, after which the excommunication was lifted.

Theodosius died from an illness in January of 395, having founded a dynasty which was to last until 450. He was a capable military leader, but his significance to history rests primarily on two items. First, his division of east and west under his sons was to divide the empire for the last time - though there were diplomatic ties, the empire was never rejoined. Second, his policy of exterminating the "heretical" Christian sects as well as pagan centers of worship ensured the ascendance of Nicene Christianity as the dominant religion of Europe, through the middle ages.