Theodosius the Great was the last Roman Emperor to rule a united Roman Empire. He established Christianity as the state religion and successfully defended it against paganism and Arianism. He founded a dynasty which ruled in the east and west until 450 and 455 respectively.
Born in Spain, Flavius Theodosius was the son of Count Flavius Theodosius, a Hispano-Roman general of Valentinian I. The younger Theodosius gained military experience in his father's campaigns in Britain and later became an accomplished military commander on the Danube River. His martial reputation was established by his victories over non-Germanic peoples who had filtered into the Yugoslavian Danube area. He was appointed (375) military governor of Moesia but was forced to retire from public life following the politically motivated execution (376) of his father.
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The new emperor, Valentinian's son Gratian, brought Theodosius back from retirement to settle a Balkan crisis in the Eastern Empire. Visgoths, seeking refuge within the Empire, attacked and defeated the emperor Valens in a great battle at Adrianople in Thrace (378). Theodosius was given military command and restored order by defeating the barbarians.
Theodosius was named emperor of the East in January 379. He spent the next few years on military campaign against the Goths with inconclusive results. In October 382, Theodosius realized that victory was not attainable and consequently a settlement was reached. Unlike previous settlements with barbarians, the Goths were permitted to live in Thrace, within the Empire. They were obligated to military service. However they retained their ruler and laws. The unintended result was the creation of a separate nation within the Empire which remained a persistent danger to its internal stability. Some modern historians have intimated that this threat was a factor in the eventual decline of the Western Empire.
The Eastern Empire was strengthened with the creation of separate military commands reporting directly to the Emperor. This effectively prevented any one individual from usurping power from rightful authority, as was possible in the West.
While fighting against barbarians, Theodosius was involved in religious matters. He was baptized in 380, after a severe illness and near death. He may have had a conversion experience which caused him to oppose powerful Arian factions who denied the divinity of the Son as stated in the Nicene Creed. Theodosius replaced the Arian bishop Demophilus with an orthodox Catholic bishop Gregory Nazianzus, in November 380. In February 380, he had decreed that all religious and others accept the Trinity.
Theodosius was more moderate towards paganism in his early reign, but eventually he ordered all temples and churches be given over to the authority of Nicene Catholics. In 381, he organized the Council of Constantinople, where 150 eastern bishops attested to the divinity of the Holy Spirit and the Son of God.
He initiated Valentinian II as emperor of the West in 388, who converted from Arianism to Catholicism. Theodosius was under the influence of Bishop Ambrose of Milan and became closer to the Church. Following the reprisal massacre of civilians (390) in Thessalonica, Ambrose compelled Theodosius to humble himself publicly and submit himself to the authority of the Church.
During his reign, Theodosius successfully fought two brutal civil wars against usurpers Maximus and Eugenius. As the defender of Christianity (394), Theodosius defeated pagan forces under Arbogast and Eugenius. This is considered by many to be his greatest achievement. Upon his death in 395, his sons Arcadius and Honorius ruled the East and West respectively. Theodosius the Great brought peace to the east with diplomacy and defended it with a rebuilt military. He won the people's favor by establishing Constantinople as the city of beauty and plenty. The dynasty that ruled after his death ensured the continuity and emergence of Nicene Christianity which lasted throughout the middle ages.