The Ecclesiastical History of Socrates Scholasticus

Gratian being now in possession of the empire, together with Valentinian the younger, and condemning the cruel policy of his uncle Valens towards the [orthodox] Christians, recalled those whom he had sent into exile. He moreover enacted that persons of all sects, without distinction, might securely assemble together in their churches; and that only the Eunomians, Photinians, and Manichæans should be excluded from the churches. Being also sensible of the languishing condition of the Roman empire, and of the growing power of the barbarians and perceiving that the state was in need of a brave and prudent man, he took Theodosius as his colleague in the sovereign power. This [Theodosius] was descended from a noble family in Spain, and had acquired so distinguished a celebrity for his prowess in the wars, that he was universally considered worthy of imperial dignity, even before Gratian’s election of him. Having therefore proclaimed him emperor at Sirmium a city of Illyricum in the consulate of Ausonius and Olybrius, on the 16th of January, he divided with him the care of managing the war against the barbarians.