The Western Roman emperor desperately needed his talents in 378 c.e., when the German Goths defeated the Roman army and killed the Eastern Roman emperor at the Battle of Adrianople and overran the Balkans. Theodosius was recalled to military command and promoted to Eastern Roman emperor in 379.
He recruited a new army (including many Germans), fought the Goths, and made peace by settling them as independent allies (foederati) inside the empire. They ruled their own people, provided military service, and received positions in the empire’s military command.
It was a precarious situation, but it restored a level of order and security. Theodosius’s military ability was also demonstrated by two victorious campaigns against western usurpers in 384 and 394.
In the latter campaign, the usurper murdered the Western Roman emperor but was defeated by Theodosius, which left him sole emperor for the entire empire from Britain to eastern Anatolia. This was the last occasion when a single emperor ruled both halves of the Roman Empire. Theodosius governed not from Rome in the West but at Constantinople in the wealthier and more populous East.
Theodosius was a zealous Christian. When he fell ill in 380 and believed that he was near death, he was baptized. It was not uncommon to wait until just before death for baptism in order to wash away sins. However, he recovered unexpectedly, becoming the first emperor to reign as a full member of the church. This gave bishops tremendous influence during his reign.
When Christians destroyed a Jewish synagogue in an Eastern city, Theodosius ordered the local bishop to pay for its restoration. Ambrose was appalled, believing that this demonstrated the triumph of Judaism over Christianity. He demanded the emperor rescind his order if he wanted to stay in good standing with the church.
The emperor yielded. In 390, after Theodosius had massacred several thousand citizens in Thessalonica for the murder of his military governor, Ambrose threatened Theodosius with excommunication. The emperor again yielded, publicly repenting for his action.
When Theodosius had first reached the East, he found the church struggling against Arianism, even though Arian theology had been condemned at the Council of Nicaea (325). Theodosius expelled Arian clergy in Constantinople and firmly stood by the Orthodox Church, including Patriarch Gregory Nazianzus.
He called the Second Ecumenical Council that met in Constantinople in 381 and ended the Arian threat. Theodosius also supported the church by legislating against non-Christians. He closed pagan temples, banned pagan sacrifices, ended the pagan Olympic Games, and declared Orthodox Christianity the official religion of the empire.
Henceforth, loyalty to the emperor was determined by adherence to his theological position. Upon his death in 395 his younger son Honorius ruled in the West, while his older son Arcadius ruled in the East. The dynasty of Theodosius ruled the empire until at least 450.