A barbarian, named Gaïnas, who had taken refuge among the Romans, and who had risen from the lowest ranks of the army to military command, formed a design to usurp the throne of the Roman Empire. With this in view, he sent for his countrymen, the Goths, from their own homes to come to the Roman territories, and appointed his relatives to be tribunes and chiliarchs. Tirbingilus, a relative of his, who commanded a large body of troops in Phrygia, commenced an insurrection; and to all persons of judgment it was patent that he was preparing the way. Under the pretext of resenting the devastation of many of the Phrygian cities, which had been committed to his superintendence, Gaïnas turned to their assistance; but on his arrival, when a multitude of barbarians had been equipped for war, he disclosed his plan which he had previously concealed, and pillaged the cities which he had been commanded to guard, and was about to attack others. He then proceeded to Bithynia, and encamped in the boundaries of Chalcedon, and threatened war. The cities of the East of Asia, and as many as lived between these regions and about the Euxine, being thus in danger, the emperor and his counsellors judged that it would not be safe to venture into any hazardous undertaking without preparation against men who were already desperate; for the emperor declared that he was ready to be favorable to him in every point, and sent to Gaïnas to offer him whatever he might demand.
Gaïnas requested that two consuls, named Saturninus and Aurelian, whom he suspected of being inimical, should be delivered up to him; and when they were in his power, he pardoned them. He afterwards held a conference with the emperor near Chalcedon, in the house of prayer in which the tomb of Euphemia the martyr is deposited; and after he and the emperor had mutually bound themselves by vows of friendship to each other, he threw down his arms, and repaired to Constantinople, where, by an imperial edict, he was appointed general of the infantry and cavalry. Prosperity so far beyond his deserts was more than he could bear with moderation; and as, contrary to all expectations, he had succeeded so wonderfully in his former enterprise, he determined to undermine the peace of the Catholic Church. He was a Christian, and, like the rest of the barbarians, had espoused the Arian heresy. Urged either by the presidents of this party, or by the suggestions of his own ambition, he applied to the emperor to place one of the churches of the city in the hands of the Arians. He represented that it was neither just nor proper that, while he was general of the Roman troops, he should be compelled to retire without the walls of the city when he wished to engage in prayer. John did not remain inactive when made acquainted with these proceedings. He assembled all the bishops who were then residing in the city, and went with them to the palace. He spoke at great length in the presence of the emperor and of Gaïnas, reproached the latter with being a stranger and a fugitive, and reminded him that his life had been saved by the father of the emperor, to whom he had sworn fidelity, as likewise to his children, to the Romans, and to the laws which he was striving to make powerless. When he had made this speech he showed the law which Theodosius had established, forbidding the heterodox to hold a church within the walls. Then, addressing himself to the emperor, John exhorted him to maintain the laws which had been established against heretics; and told him that it would be better to be deprived of the empire, than to be guilty of impiety by becoming a traitor to the house of God. Thus did John speak boldly like a man, and gave no place to innovation in the churches under his care. Gaïnas, however, regardless of his oaths, attacked the city. His enterprise was pre-announced by the appearance of a comet directly over the city; this comet was of extraordinary magnitude, larger, it is said, than any that had previously been seen, and reaching almost to the earth itself. Gaïnas intended to seize first upon the stores of the bankers, and hoped to collect together their enormous wealth. But since the rumor of his plan was spread, the bankers concealed their ready wealth and no longer set forth silver upon the tables, as they were wont publicly to do. Gaïnas then sent some of the barbarians by night to set fire to the palace; but they were unskillful and overcome with fear, so they turned back. For when they drew near the edifice, they fancied that they saw a multitude of heavily armed men of immense stature, and they returned to inform Gaïnas that fresh troops had just arrived. Gaïnas disbelieved their report, for he was confident that no troops had entered the city. As, however, other individuals whom he despatched to the palace for the same purpose, on the following night, returned with the same report, he went out himself to be an eye-witness of the extraordinary spectacle. Imagining that the army before him consisted of soldiers who had been withdrawn from other cities, and that these troops protected the city and palace by night and concealed themselves by day, Gaïnas feigned to be possessed of a demon; and under the pretext of offering up a prayer, went to the church which the father of the emperor had erected in honor of John the Baptist, at Hebdomos. Some of the barbarians remained in Constantinople, and others accompanied Gaïnas; they secretly carried arms and pots full of darts in the women's chariots, but when they were discovered, they slew the guard at the gates, who attempted to hinder the carrying out of the arms. From this the city was filled with as much confusion and uproar, as if it had suddenly been captured. A good thought ruled this terrible moment; for the emperor without delay declared Gaïnas a public enemy, and commanded that all the barbarians left in the city should be slain. No sooner was this mandate issued, than the soldiers rushed upon the barbarians, and slew the greater number of them; they then set fire to the church which was named after the Goths; for as was customary, they had congregated there in the house of prayer, because there was no other refuge, since the gates were shut. On hearing of this calamity, Gaïnas passed through Thrace, and proceeded towards the Cherronesus, intending to cross the Hellespont; for he thought that if he could conquer the opposite coast of Asia, he could easily subjugate to himself all the provinces of the empire in the East. All these things proved contrary to his hopes, because the Romans were there favored by Divine power. For the army sent by the emperor was on hand by land and by sea, under the command of Flavita, who although a barbarian by birth, was a good man, and an able general. The barbarians, having no ships, imprudently attempted to cross the Hellespont to the opposite continent on rafts; when suddenly a great wind blew and violently separated them, and drove them against the Roman vessels. The greater part of the barbarians and their horses were drowned; but many were slain by the military. Gaïnas, however, with a few of his followers escaped; but not long after, when fleeing through Thrace, they fell in with another detachment of the Roman army, and Gaïnas, with all his barbarians, perished. Such was the termination of the daring schemes and life of Gaïnas.
Flavita had rendered himself very conspicuous in this war, and was therefore appointed consul. During his consulate, and that of Vincentius, a son was born to the emperor. The young prince was named after his grandfather, and at the commencement of the next consulate, was proclaimed Augustus.