He came from a family of a valued Roman general of the same name, who for unknown reasons fell into disgrace and was sentenced to death. At that moment the military career which had started under the auspices of his father seemed to come to an abrupt end. Theodosius removed himself from political life and would have probably led a calm, rural life in his lands on the Pyrenean Peninsula if the Goths had not seriously threatened the empire. The Battle of Adrianople (378) was the greatest defeat of the Roman army since the times of Hannibal (213 B.C.). Emperor Valens was killed, while the empire seemed to find itself at the edge of a downfall. The moment of glory arrived for Theodosius. In the year 379 he became the August of the Eastern Empire, while in the year 394, few months prior to his death he ruled in the whole empire.

After assuming power (initially with Gratian and Valentinian III) one of the first steps of Theodosius was  elevating his father, as was the imperial custom, to the status of a venerated deity (divus) and taking on the title of the high priest – pontifex maximus. This is important, because he would be the first emperor to give up this title. The empire, was at that time divided among supporters of the old Roman beliefs (the so-called pagans), Christians and he faithful of other religions.

However, let us start at the beginning, meaning the Edict of Milan from 313, in which all faiths of the Roman Empire, both in the West and in the East were guaranteed religious tolerance. However, even during the reign of Constantine the Great, co-author of the edict, the Christians were divided. Some approved of the decisions made at the Council of Nicaea (325), while others supported the theses of Arius (Arianism). The sympathies of subsequent emperors generally oscillated around Arianism, but as a rule there were no significant religious conflicts between these religious factions. The situation changed when the energetic Ambrose became the bishop of Milan, a staunch supporter of the agreements of Nicaea and an enemy of the Arians. And he was the one who became the main driving force behind the religious policy during the reign of Theodosius. One year after assuming power (380), the emperor declared war on the Arians. At the behest of Pope Damasus I and bishop Ambrose he issued an edict which proclaimed the Nicene profession as obligatory, stigmatizing at the same time other ones (e.g. Manicheism), but mainly Arianism, as heretical. According to the law, all religious deviations were to be persecuted by the state, while the properties of heretical communes were to be confiscated and Arian priests were not to be ordained. This was confirmed by a general council called in the year 381 in Constantinople. Of course, we could say that the goal of such decrees was striving for religious unity in the empire and peace among the rivaling factions, but that was not all.

As opposed to previous emperors, who generally permitted themselves to be baptized at the end of their life, Theodosius did so still as a general at the time of a serious illness which befell him in the year 380. As it would later turn out this act had great significance on his later activities. This is because, Theodosius had to be aware of the religious sanctions which could be placed upon him in case of any conflict with the Church. An example of this is a situation which took place in the year 388 in the Eastern Empire. When in the city of Callinicum on the Euphrates a synagogue was burned down and Jews were killed by the local Christians, Theodosius ordered the perpetrators to be punished, while the synagogue was to be rebuilt. However, he encountered strong resistance from bishop Ambrose, who believed that such a form of imperial intervention would be proof of going against one’s faith. When the emperor attempted to oppose the bishop’s will, he threatened him with not holding mass in his name and not giving him the sacraments. How this whole affair ended we can only assume. Two years later in Thessaloniki, there was a massacre of several thousand inhabitants as part of a punishment by the emperor, who desired to discipline the city after riots were incited and a military leader was killed. The bishop of Milan insulted by this fact refused Theodosius participation in a mass and only after the emperor had repented he was allowed to take the Holy Communion on Christmas Day.

Another front of the religious struggle was the traditional beliefs of Romans, meaning paganism. It especially affected Rome itself – a city which to a large extent remained faithful to the old cults. The conflict erupted at a time, when senators desired to bring back a statue which had been previously removed,   of the victorious Victoria, brought to Rome by Octavius Augustus after winning the Battle of Actium. It was a symbol of the greatness of the empire and it was believed that as long as it watched over the city it would remain safe. Ultimately it was not reintroduced into the Senate, while the emperor began a radical campaign against traditional pagan beliefs. With this goal in mind he issued decrees between the years 391 and 393. It began with the taking away of tax exemptions from priests and the Vestals, while temples that did not possess a title to inherit the land were shut down. In the year 391 sacrifices to the old gods were forbidden while pagan temples could not be approached – and this is the year which is considered to be the beginning of Christianity as a state religion. A year later this ban was strengthened announcing the death penalty to all those who would not abide by this law. At the same time it was forbidden to celebrate pagan cults at home. All religious symbols (lamps, altars) were to be removed wreaths could not be hung, incense burners were forbidden as well. Snitches were seen throughout the city, while the punishment for insubordination was in the very least a few kilograms of gold. However, the emperor never personally ordered the destruction of pagan temples, such orders came from local governors or bishops. Pagans played a significant role in the army, politics, culture, and administration and still constituted, at least in the West, a large majority, with which the emperor, whether he wanted to or not, had to contend with.

The anti-pagan attitude of Theodosius was of course connected with the influence, which representatives of the Church had over him, but was also the result of political reasons. After the death of Valentinian II, a certain Eugenius appointed himself as the ruler of the Western Empire, a faithful of the old Roman cults. In the years 394-395 he was defeated by Theodosius. However, it must be admitted that after killing Eugenius and putting down the “pagan” revolt with which most of the senators sympathized, the emperor did not repress them in any way and aimed at reconciliation.

Another grand initiative of Theodosius, which in a decisive way impacted the military condition of the empire, was the creation of autonomous regiments, the so-called foederati, which recruited themselves from among Goth warriors and were headed by Goth leaders, who were subject to Roman superiors. These warriors, officially called allies of Rome, were guaranteed the right to settle on imperial land, however they did not enjoy the same rights as Romans. They were not allowed to marry with Roman citizens. The senators of that time heavily criticized the rights which were guaranteed to Goths. These warriors, while strengthening the army with battle-ready regiments also as it turned out would create quite a problem in the near future. Theodosius enjoyed the respect of the Goths and until his death they did not constitute a threat to the empire, respecting the agreements in place. This situation changed when the emperor died. 

Theodosius was only ruler of the unified empire for a short time. However, he did not take advantage of the opportunity for long-lasting unification of both parts, which also had important political repercussions. Even prior to his premature death in the year 395 he divided the state between his underage sons – Arcadius and Honorius (Theodosian dynasty). The former received the eastern part, while the latter – the western. The emperor could most likely not foresee that this step would bring about a lengthy period of division between both parts – which had initially cooperated with each other, but later more often saw each other as rivals. Besides the aforementioned sons  Theodosius also had two ambitious and politically savvy daughters – Pulcheria and Galla Placidia.

After his death, the body of the emperor was taken to Constantinople and laid to rest in the Basilica of the Apostles.