In 379, after the eastern Roman emperor Valens was killed at the Battle of Adrianople against the Goths, Gratian appointed Theodosius as a successor with orders to take charge of the military emergency.

Although Theodosius interfered little in the functioning of traditional pagan cults and appointed non-Christians to high offices, he failed to prevent or punish the damaging of several Hellenistic temples of classical antiquity, such as the Serapeum of Alexandria, by Christian zealots.

[10] Theodosius sponsored several measures to improve his capital and main residence, Constantinople, most notably his expansion of the Forum Tauri, which became the biggest public square known in antiquity.

[19] His father of the same name, Count Theodosius, was a successful and high-ranking general (magister equitum) under the western Roman emperor Valentinian I, and his mother was called Thermantia.

[30] Not long afterwards, however, under mysterious circumstances, Theodosius's father suddenly fell from imperial favor and was executed, and the future emperor felt compelled to retire to his estates in Hispania.

[33] Theodosius's period away from service in Hispania, during which he was said to have received threats from those responsible for his father's death,[34] did not last long, however, as Maximinus, the probable culprit, was himself removed from power around April 376 and then executed.

[36][iv] Theodosius's renewed term of office seems to have gone uneventfully,[41] until news arrived that the eastern Roman emperor, Valens, had been killed at the Battle of Adrianople in August 378 against invading Goths.

The disastrous defeat left much of Rome's military leadership dead, discredited, or barbarian in origin, to the result that Theodosius, notwithstanding his own modest record, became the establishment's choice to replace Valens and assume control of the crisis.

[43] The immediate problem facing Theodosius upon his accession was how to check the bands of Goths that were laying waste to the Balkans, with an army that had been severely depleted of manpower following the debacle at Adrianople.

[44] The western emperor Gratian, who seems to have provided only little immediate assistance,[45] surrendered to Theodosius control of the praetorian prefecture of Illyricum for the duration of the conflict, giving his new colleague full charge the war effort.

[66] Theodosius I was based in Constantinople, and according to Peter Heather, wanted, "for his own dynastic reasons (for his two sons each eventually to inherit half of the empire), refused to appoint a recognized counterpart in the west.

As a result he was faced with rumbling discontent there, as well as dangerous usurpers, who found plentiful support among the bureaucrats and military officers who felt they were not getting a fair share of the imperial cake.

[74] Now the de facto ruler of the Western empire as well, Theodosius celebrated his victory in Rome on 13 June 389 and stayed in Milan until 391, installing his own loyalists in senior positions including the new magister militum of the West, the Frankish general Arbogast.

[75]: 223  Several scholars, such as historian G. W. Bowersock and authors Stephen Williams and Gerard Friell, think that Theodosius ordered the massacre in an excess of "volcanic anger".

[87][86]: 110  Doleźal says Sozomen is very specific in saying that in response to the riot, the soldiers made random arrests in the hippodrome to perform a few public executions as a demonstration of imperial disfavor, but the citizenry objected.

Doleźal suggests, "The soldiers, realizing that they were surrounded by angry citizens, perhaps panicked ... and ... forcibly cleared the hippodrome at the cost of several thousands of lives of local inhabitants".

[76]: 103–104  McLynn says Theodosius was “unable to impose discipline upon the faraway troops" and covered that failure by taking responsibility for the massacre on himself, declaring he had given the order then countermanded it too late to stop it.

[88]: 262–263 Washburn says the image of the mitered prelate braced in the door of the cathedral in Milan blocking Theodosius from entering is a product of the imagination of Theodoret who wrote of the events of 390 "using his own ideology to fill the gaps in the historical record".

[106][107] Historian Michele Renee Salzman explains that "two newly relevant texts – John Chrysostom's Homily 6, adversus Catharos (PG 63: 491–492) and the Consultationes Zacchei et Apollonii, re-dated to the 390s, reinforces the view that religion was not the key ideological element in the events at the time".

[108] According to Maijastina Kahlos, Finnish historian and Docent of Latin language and Roman literature at the University of Helsinki, the notion of pagan aristocrats united in a "heroic and cultured resistance" who rose up against the ruthless advance of Christianity in a final battle near Frigidus in 394 is a romantic myth.

A sixth-century source puts the raising of the obelisk in the year 390, and Greek and Latin epigrams on the plinth (the lower part of the base) credit Theodosius I and the urban prefect Proclus with this feat.

[136]: 20 On 28 February 380, Theodosius issued the Edict of Thessalonica, a decree addressed to the city of Constantinople, determining that only Christians who believed in the consubstantiality of God the Father, Son and Holy Spirit could style themselves "catholic" and have their own places of worship officially recognized as "churches"; deviants were labeled heretics and described as "out of their minds and insane".

[137][138][v] Recent scholarship has tended to reject former views that the edict was a key step in establishing Christianity as the official religion of the Empire, since it was aimed exclusively at Constantinople and seems to have gone largely unnoticed by contemporaries outside the capital.

"[150] Theodosius seems to have adopted a cautious policy toward traditional non-Christian cults, reiterating his Christian predecessors' bans on animal sacrifice, divination, and apostasy, while allowing other pagan practices to be performed publicly and temples to remain open.

[151][152][153] He also voiced his support for the preservation of temple buildings, but nonetheless failed to prevent the damaging of many holy sites, images and objects of piety by Christian zealots, some including even his own officials.

[169]: 47  French historian of Antiquity, Philippe Fleury [fr], observes that Ammianus Marcellinus says this legal complexity produced corruption, forgery of rescripts, falsified appeals, and costly judicial delays.

[183] Brown says the language of these laws is uniformly vehement, and penalties are harsh and frequently horrifying, leading some historians, such as Ramsay MacMullen, to see them as a 'declaration of war' on traditional religious practices.

[200]: 2 Scholars agree that Theodosius gathered copious legislation on religious subjects, and that he continued the practices of his predecessors, prohibiting sacrifices with the intent of divining the future in December of 380, issuing a decree against heretics on 10 January 381, and an edict against Manichaeism in May of that same year.

[180]: 5–16, 95  Previously undervalued similarities in language, society, religion, and the arts, as well as current archaeological research, indicate paganism slowly declined, and that it was not forcefully overthrown by Theodosius I in the fourth century.

[203]: xv Maijastina Kahlos writes that the fourth century Roman empire contained a wide variety of religions, cults, sects, beliefs and practices and they all generally co-existed without incident.