The Ecclesiastical History
Hermias Sozomen

Book I
Chapter V
Crispus, on whose account, it is said, Constantine required purification, did not die till the twentieth year of his father's reign; he held the second place in the empire and bore the name of Caesar and many laws, framed with his sanction in favor of Christianity, are still extant. That this was the case can be proved by referring to the dates affixed to these laws, and to the lists of the legislators. It does not appear likely that Sopater had any intercourse with Constantine whose government was then centered in the regions near the ocean and the Rhine; for his dispute with Maxentius, the governor of Italy, had created so much dissension in the Roman dominions, that it was then no easy matter to dwell in Gaul, in Britain, or in the neighboring countries, in which it is universally admitted Constantine embraced the religion of the Christians, previous to his war with Maxentius, and prior to his return to Rome and Italy: and this is evidenced by the dates of the laws which he enacted in favor of religion.

Chapter VI
Hence it is probable that while Constantius was alive, it did not seem contrary to the laws for the inhabitants of the countries beyond Italy to profess Christianity, that is to say, in Gaul, in Britain, or in the region of the Pyrenean mountains as far as the Western Ocean.

Book II
Chapter VI
The church having been in this manner spread throughout the whole Roman world, religion was introduced even among the barbarians themselves. The tribes on both sides of the Rhine were Christianized, as likewise the Celts and the Gauls who dwelt upon the most distant shores of the ocean; the Goths, too, and such tribes aswere contiguous to them, who formerly dwelt on either of the high shores of the Danube, had long shared in the Christian faith, and had changed into a gentler and more rational observance. Almost all the barbarians had professed to hold the Christian doctrine in honor, from the time of the wars between the Romans and foreign tribes, under the government of Gallienus and the emperors who succeeded him. For when an unspeakable multitude of mixed nations passed over from Thrace into Asia and overran it, and when other barbarians from the various regions did the same things to the adjacent Romans, many priests of Christ who had been taken captive, dwelt among these tribes; and during their residence among them, healed the sick, and cleansed those who were possessed of demons, by the name of Christ only, and by calling on the Son of God; moreover they led a blameless life, and excited envy by their virtues. The barbarians, amazed at the conduct and wonderful works of these men, thought that it would be prudent on their part, and pleasing to the Deity, if they should imitate those whom they saw were better; and, like them, would render homage to God. When teachers as to what should be done, had been proposed to them, the people were taught and baptized, and subsequently were gathered into churches.

Chapter XXVIII
The emperor, either believing their statements to be true, or imagining that unanimity would be restored among the bishops if Athanasius were removed, exiled him to Treves, in Western Gaul; and thither, therefore, he was conducted.

Book III
Chapter II
At this period Athanasius returned from Gaul to Alexandria. It is said that Constantine intended to have recalled him, and that in his testament he even gave orders to that effect. But as he was prevented by death from performing his intention, his son who bore his name, and who was then commanding in Western Gaul, recalled Athanasius, and wrote a letter on the subject to the people of Alexandria.

Chapter VI
At the same period of time, the Franks devastated Western Gaul; and the provinces of the East, and more particularly Antioch after the Synod, were visited by tremendous earthquakes.

Chapter XI
Maximus, bishop of Treves, was deposed, because he had been among the first who had received Paul into communion, and had been the cause of his returning to Constantinople, and because he had excluded from communion the Eastern bishops who had repaired to Gaul.

Book IV
Chapter I
Four years after the council of Sardica, Constans was killed in Western Gaul. Magnentius, who had plotted his murder, reduced the entire government of Constans under his own sway. In the meantime Vetranio was proclaimed emperor at Sirmium, by the Illyrian troops. Nepotian, the son of the late emperor's sister, gathered about him a body of gladiators, and wrangled for the imperial power, and ancient Rome had the greatest share of these evils. Nepotian, however, was put to death by the soldiers of Magnentius. Constantius, finding himself the sole master of the empire, was proclaimed sole ruler, and hastened to depose the tyrants. In the meantime, Athanasius, having arrived in Alexandria, prepared to convene a Synod of the Egyptian bishops, and had the enactments confirmed which had been passed at Sardica, and in Palestine, in his favor.

Chapter VII
In the meantime, Magnentius made himself master of ancient Rome, and put numbers of the senators, and of the people, to death. Hearing that the troops of Constantius were approaching, he retired into Gaul; and here the two parties had frequent encounters, in which sometimes the one and sometimes the other was victorious. At length, however, Magnentius was defeated, and fled to Mursa, which is the fortress of this Gaul, and when he saw that his soldiers were dispirited because they had been defeated, he stood on an elevated spot and endeavored to revive their courage. But, although they addressed Magnentius with the acclamations usually paid to emperors, and were ready to shout at his public appearance, they secretly and without premeditation shouted for Constantius as emperor in place of Magnentius. Magnentius, concluding from this circumstance, that he was not destined by God to hold the reins of empire, endeavored to retreat from the fortress to some distant place. But he was pursued by the troops of Constantius, and being overtaken at a spot called Mount Seleucus, he escaped alone from the encounter, and fled to Lugduna. On his arrival there, he slew his own mother and his brother, whom he had named Caesar; and lastly, he killed himself. Not long after, Decentius, another of his brothers, put an end to his own existence. Still the public tumults were not quelled; for not long after, Silvanus assumed the supreme authority in Gaul; but he was put to death immediately by the generals of Constantius.

Book V
Chapter II
Eusebia, the wife of Constantius, obtained for him permission to retire to Athens; and he accordingly settled there, under pretext of attending the pagan exercises and schools; but as rumor says, he communed with diviners concerning his future prospects. Constantius recalled him, and proclaimed him Caesar, promised him his sister Constantia in marriage, and sent him to Gaul; for the barbarians whose aid had been hired by Constantius previously against Magnentius, finding that their services were not required, had portioned out that country. As Julian was very young, generals, to whom the prudential affairs were turned over, were sent with him; but as these generals abandoned themselves to pleasure, he was present as Caesar, and provided for the war. He confirmed his soldiers in their spirit for battle, and urged them in other ways to incur danger; he also ordered that a fixed reward should be given to each one who should slay a barbarian. After he had thus secured the affections of the soldiery, he wrote to Constantius, acquainting him with the levity of the generals; and when another general had been sent, he attacked the barbarians, and obtained the victory. They sent embassies to beg for peace, and showed the letter in which Constantius had requested them to enter the Roman dominions. He purposely delayed to send the ambassador back; he attacked a number of the enemy unexpectedly and conquered them.

Book VI
Chapter VI
When Julian was in Gaul, he went one day to a temple to offer incense. Valentinian16 accompanied him, according to an ancient Roman law, which still prevails, and which enacted that the leader of the Jovians and the Herculeans (that is to say, the legions of soldiers who have received this appellation in honor of Jupiter and of Hercules) should always attend the emperor as his body-guard. When they were about to enter the temple, the priest, in accordance with the pagan custom, sprinkled water upon them with the branch of a tree. A drop fell upon the robe of Valentinian; he scarcely could restrain himself, for he was a Christian, and he rebuked his asperser; it is even said that he cut off, in view of the emperor, the portion of the garment on which the water had fallen, and flung it from him. From that moment Julian entertained inimical feelings against him, and soon after banished him to Melitine in Armenia, under the plea of misconduct in military affairs; for he would not have religion regarded as the cause of the decree, lest Valentinian should be accounted a martyr or a confessor.

Chapter X
When the deputies arrived in Italy, they found that the emperor was in Gaul, engaged in war against the barbarians. As they considered that it would be perilous to visit the seat of war in Gaul, they delivered their letter to Liberius.

Chapter XXIII
Auxentius and his followers differed from the others in opinion; he was then president of the church in Milan, and, in conjunction with a few partisans, was intent upon the introduction of innovations, and the maintenance of the Arian dogma of the dissimilarity of the Son and of the Holy Ghost, according to the inquiry which had last sprung up, in opposition to the unanimous agreement of the Western priests. The bishops of Gaul and of Venetia having reported that similar attempts to disturb the peace of the Church were being made by others among them, the bishops of several provinces assembled not long after at Rome, and decreed that Auxentius and those who held his sentiments should be aliens from their communion. They confirmed the traditional faith established by the council of Nicaea, and annulled all the decrees that had been issued at Ariminum contrary to that faith, under the plea that these decrees had not received the assent of the bishop of Rome, nor of other bishops who agreed with them, and that many who had been present at the Synod, had disapproved of the enactments there made by them.

Chapter XXXVI
The Sarmatians having invaded the western parts of the empire, Valentinian levied an army to oppose them. As soon, however, as they heard of the number and strength of the troops raised against them, they sent an embassy to solicit peace. When the ambassadors were ushered into the presence of Valentinian, he asked them whether all the Sarmatians were similar to them. On their replying that the principal men of the nation had been selected to form the embassy, the emperor exclaimed, in great fury, "A terrible thing do our subjects endure, and a calamity is surrounding the Roman government, if the Sarmatians, a barbarous race, of whom these are your best men, do not love to abide by themselves, but are emboldened to invade my government, and presume to make war at all against the Romans." He spoke in this strain for some time in a very high pitch of voice, and his rage was so violent and so unbounded, that at length he burst simultaneously a blood-vessel and an artery. He lost, in consequence, a great quantity of blood, and expired soon after in a fortress of Gaul.

Book VII
Chapter XIII
As the Emperor Gratian was at this period occupied with a war against the Alamanni,25 Maximus quitted Britain, with the design of usurping the imperial power. Valentinian was then residing in Italy, but as he was a minor, the affairs of state were transacted by Probus, a praetorian prefect, who had formerly been consul.

Book IX
Chapter XI
During this period many tyrants rebelled against Honorius in the Western government. Some fell upon one another, while others were apprehended in a marvelous way, and so evidenced that the Divine love toward Honorius was not common. The soldiers in Britain were the first to rise up in sedition, and they proclaimed Mark as tyrant. Afterwards, however, they slew Mark, and proclaimed Gratian. Within four months subsequently they killed Gratian, and elected Constantine in his place, imagining that, on account of his name, he would be able to reduce the empire firmly under his authority; and for no other reason than this, several other persons of the same name were elected to the tyranny. Constantine passed over from Britain to Bononia, a maritime city of Gaul; and after inducing all the troops in Gaul and Aquitania to espouse his cause, he reduced to obedience the inhabitants of the regions extending to themountains which divide Italy from Gaul, and which the Romans have named the Cottian Alps. He then sent his oldest son, Constans, whom he had already nominated Caesar, and whom he afterwards proclaimed emperor, into Spain. Constans, after making himself master of this province, and appointing his own governors over it, commanded that Didymus and Verinian, relatives of Honorius, should be loaded with chains, and brought before him. Didymus and Verinian had at first differed among themselves, but a reconciliation was effected between them, when they found themselves menaced by the same danger. They combined their forces, which consisted. chiefly of armed peasants and slaves. They attacked Lusitania in common, and slew many of the soldiers sent by the tyrant for their capture.

SOURCE
Sozomenus. Church History from A.D. 323-425. Nicene and Post-Nicene Fathers, Series II, Vol. II. URL: http://www.ccel.org/fathers2/NPNF2-02/TOC.htm