Date of birth uncertain; d. about A.D. 392. He was of noble family, probably of Antioch. St. Basil calls him a "nursling" of Silvanus, Bishop of Tarsus, but whether this discipleship was at Antioch or at Tarsus is not known. He studied at Athens, then embraced the monastic state. He became head of a monastery in or near Antioch, and St. Chrysostom was his disciple. When Antioch groaned under Arian bishops, he did not join the small party of irreconcilables headed by Paulinus, yet when Bishop Leontius made Aëtius a deacon Diodorus and Flavian threatened to leave his communion and retire to the West, and the bishop yielded. These two holy men, though not priests taught the people to sing the Psalms in alternate choirs (a practice which quickly spread throughout the Church ), at first in the chapels of the martyrs, then, at Leontius's invitation, in the churches. When at length, in 361, the Arian party appointed an orthodox bishop in the person of St. Meletius, Diodorus was made priest. He seems to have written some of his works against the pagans as early as the reign of Julian, for that emperor declared that Diodorus had used the learning and eloquence of Athens against the immortal gods, who had punished him with sickness of the throat, emaciation, wrinkles, and a hard and bitter life. In the persecution of Valens (364-78), Flavian and Diodorus, now priests, during the exile of Meletius kept the Catholics together, assembling them on the northern bank of the Orontes, since the Arian emperor did not permit Catholic worship within the city. Many times banished, Diodorus, in 372, made the acquaintance of St. Basil in Armenia, whither that saint had come to visit Meletius. On the return of the latter to his flock, he made Diodorus Bishop of Tarsus and Metropolitan of Cilicia. Theodosius soon after, in a decree, named Diodorus and St. Pelagius of Laodicea as norms of orthodoxy for the whole East. Diodorus was at the Councils of Antioch in 379 and of Constantinople in 381. Sozomen makes him responsible at the latter council for the proposal of Nectarius as bishop of that city, and represents him as one of the chief movers in the appointment of St. Flavian as successor to Meletius, by which the unhappy schism at Antioch was prolonged.
Diodorus came to Antioch in 386 or later, when St. Chrysostom was already a priest. In a sermon he spoke of Chrysostom as a St. John the Baptist, the Voice of the Church, the Rod of Moses. Next day Chrysostom ascended the pulpit and declared that when the people had applauded, he had groaned; it was Diodorus, his father, who was John the Baptist, the Antiochenes could bear witness how he had lived without possessions, having his food from alms, and persevering in prayer and preaching; like the Baptist he had taught on the other side of the river, often he had been imprisoned --nay, he had been often beheaded, at least in will, for the Faith. In another sermon he likens Diodorus to the martyrs : "See his mortified limbs, his face, having the form of a man, but the expression of an Angel !"
St. Basil in 375 asked Diodorus to disown a fictitious letter circulated in his name, permitting marriage with a deceased wife's sister. In the following year he criticizes the rhetorical style of the longer of two treatises sent him by Diodorus, but gives warm praise to the shorter. Diodorus's style is praised by Chrysostom, Theodoret, and Photius, but of his very numerous writings of a few unimportant fragments have been preserved, chiefly in Catenæ (q.v). He wrote against some of the heresies and still more against heathen philosophy. Photius gives a detailed summary of his eight books "de Fato"; they were evidently very dull from a modern point of view. According to Leontius he composed commentaries on the whole Bible. St. Jerome says that these were imitations of those of Eusebius of Emesa, but less distinguished by secular learning. Diodorus rejected the allegorical interpretation of the Alexandrians, and adhered to the literal sense. In this he was followed by his disciple Theodore of Mopsuestia, and by Chrysostom in his unequalled expositions. The Antiochene School of which he was the leader was discredited by the subsequent heresies of Nestorius, of whom his disciple Theodore of Mopsuestia was the precursor. Theodoret wrote to exculpate Diodorus, but St. Cyril declared him a heretic. The damning passages cited by Darius Mercator and Leontius seem, however, to belong to a work of Theodore, not of Diodorus; nor was the latter condemned when Theodore and passages of Theodoret and Ibas (the Three Chapters ) were condemned by the Fifth General Council (553). It seems certain that Diodorus went too far in his opposition to (the younger) Apollinarius of Laodicea, according to whom the rational soul in Christ was supplied by the Logos. Diodorus, in emphasizing the completeness of the Sacred Humanity, appears to have asserted two hypostases, not necessarily in a heretical sense. If the developments by Theodore throw a shade on the reputation of Diodorus, the praise of all his contemporaries and especially of his disciple Chrysostom tend yet more strongly to exculpate him. It will be best to look upon Diodorus as the innocent source of Nestorianism only in the sense that St. Cyril of Alexandria is admittedly the unwilling origin of Monophysitism through some incorrect expressions. Against this view are Julicher [in Theol. lit. Z. (1902), 82-86] and Funk [in "Rev. d' hist. eccl.", III (1902), 947-71; reprinted with improvements in "Kirchengesch, Abhandl." (Paderborn, 1907), III, 323].
The fragments of his Commentaries on the Old Testament are collected in Migne, P.G., XXXIII, from the Catena of Nicephorus and that published by Corderius (Antwerp, 1643-6), also from Mai, "Nova Patrum Bibl.", VI. A few more are found in Pitra "Spicilegium Solesmense" (Paris, 1852), I. A long list of the lost works is in Fabricius, "Bibl. Gr.", V, 24 (reprinted in Migne loc. cit.). Some Syriac dogmatic fragments are in Lagarde, Analecta Syriaca", (Leipzig and London, 1858). Four treatises of Pseudo-Justin Martyr have been attributed to Diodorus by Harnack ("Texte und Unters.", N.F., VI, 4, 1901).
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