Theodosius I (Latin: Flavius Theodosius Augustus; 11 January 347 – 17 January 395), also known as Theodosius the Great, was Roman Emperor from AD 379 to AD 395. Theodosius was the last emperor to rule over both the eastern and the western halves of the Roman Empire. On accepting his elevation, he campaigned against Goths and other barbarians who had invaded the empire. He failed to kill, expel, or entirely subjugate them, and after the Gothic War, they established a homeland south of the Danube, in Illyricum, within the empire's borders. He fought two destructive civil wars, in which he defeated the usurpers Magnus Maximus and Eugenius at great cost to the power of the empire.

He also issued decrees that effectively made Orthodox Nicene Christianity the official state church of the Roman Empire. He neither prevented nor punished the destruction of prominent Hellenistic temples of classical antiquity, including the Temple of Apollo in Delphi and the Serapeum in Alexandria. He dissolved the order of the Vestal Virgins in Rome. In 393, he banned the pagan rituals of the Olympics in Ancient Greece. After his death, Theodosius' young sons Arcadius and Honorius inherited the east and west halves respectively, and the Roman Empire was never again re-united, though Eastern Roman emperors after Zeno would claim the united title after Julius Nepos' death in 480 AD.

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