Reign of Theodosius the Great; he was initiated into Divine Baptism by Ascholius, Bishop of Thessalonica. The Letters he addressed to those who did not hold the Definition of the Council of Nice
As Gaul was about this period infested by the incursions of the Alemanni, Gratian returned to his paternal dominions, which he had reserved for himself and his brother, when he bestowed the government of Illyria and of the Eastern provinces upon Theodosius. He effected his purpose with regard to the barbarians; and Theodosius was equally successful against the tribes from the banks of the Ister; he defeated them, compelled them to sue for peace, and, after accepting hostages from them, proceeded to Thessalonica.
He fell ill while in this city, and after receiving instruction from Ascholius, the bishop, he was initiated, and was soon after restored to health. The parents of Theodosius were Christians, and were attached to the Nicene doctrines; he was pleased with Ascholius, who maintained the same doctrines, and was, in a word, endowed with every virtue of the priesthood. He also rejoiced at finding that the Arian heresy had not been participated in by Illyria. He inquired concerning the religious sentiments which were prevalent in the other provinces, and ascertained that, as far as Macedonia, all the churches were like minded, and all held that equal homage ought to be rendered to God the Word, and to the Holy Ghost, as to God the Father; but that towards the East, and particularly at Constantinople, the people were divided into many different heresies.
Reflecting that it would be better to propound his own religious views to his subjects, so as not to appear to be using force by commanding the unwilling subject to worship contrary to his judgment, Theodosius enacted a law at Thessalonica, which he caused to be published at Constantinople, well knowing that the rescript would speedily become public to all the other cities, if issued from that city, which is as a citadel of the whole empire. He made known by this law his intention of leading all his subjects to the reception of that faith which Peter, the chief of the apostles, had, from the beginning, preached to the Romans, and which was professed by Damasus, bishop of Rome, and by Peter, bishop of Alexandria.
He enacted that the title of “Catholic Church” should be exclusively confined to those who rendered equal homage to the Three Persons of the Trinity, and that those individuals who entertained opposite opinions should be treated as heretics, regarded with contempt, and delivered over to punishment.
Source: Ecclesiastical History (New Advent)