Diodorus bishop of Tarsus, is supposed to have been born at Antioch. After being ordained priest there, and intrusted with the care of its Church during the banishment of Meletius, its head, though only in priest's orders, he acted so prudently and courageously as to maintain orthodoxy in the see. After the return of Meletius he was ordained bishop of Tarsus, A.D. 378. So great was his fame that he was chosen to take care of the interests of the Eastern churches at the Council of Constantinople, A.D. 381. The date of his death is not accurately known, but it must have been before A.D. 394. None of his works have come down to us except in fragments or extracts, preserved by Photius and others. He was highly esteemed by the great men of his own and after times, and his writings much commended. Theodore of Mopsuestia, who was an advocate of Nestorianism, was his pupil, and the scholar was supposed to have imbibed his heresy from his master. Chrysostom was also one of his pupils. Even the fame and orthodoxy of St. Chrysostom could not avail his former master. The loss of his works is the more to be regretted, as he was the first that began to throw aside allegory in the interpretation of Scripture. From the catalogue of his works mentioned by Suidas (in voc. Diodor.), most of them appear to have been explanations of Scripture, or controversial tracts; Photius has preserved (Cod. 223, page 662) much of his argument taken out of a treatise on Fate; and Ebedjesu (Asseman. Bib. Or. tom. 3, page 39), in his catalogue of Syriac ecclesiastical writers, mentions 60 books of Diodotus that the Arians burned, and gives the titles of eight of them. His style was clear and perspicuous, according to the testimony of Photius, and his arguments, says St. Basil (Epist. 167), were close and well arranged, expressed in language of the greatest simplicity (Socrates, Hist. Eccl. chapter 6; Theodoret, Hist. Eccl. 4:25). See the list of his writings in Fabricius, Bibliotheca Graeca (ed. Harles), 9:277-282; also Leo Allatius, Diatriba de Theodoris, No. 66, apud Ang. Mai, Biblioth. Nov. Patr. 6:137; also given in Migne, Patrologia Graeca, 33:1545-1627, where fragments of the commentaries of Diodorus on the Pentateuch and Psalms are given in Greek and Latin. Semisch (in Herzog's Real-Encyklopadie, 3:405) gives an account of the doctrinal position of Diodorus, which we condense as follows. Diodorus died not only in the odor of sanctity, but with a high reputation for orthodoxy. The Nestorian controversy, after his death, robbed him of this reputation. Some of his writings against Apollinarism involve the principles of the later Nestorianism, e.g. the πρὸς τοὺς συνουσιαστάς, and the treatise περὶ τοῦ ἁγίου πνεύματος (Phot. Bibl. Cod. 102), of the former of which there are fragments in Marius Mercator (ed. Baluze, page 349 sq.) and Leontius Byzantinus (Canisius, Lect. Antiqq. ed. Basnage, 1:591 sq.). Here Diodorus makes the Son of God twofold, viz. the Logos of God and the Son of David, of whom the latter, not the former, was conceived by Mary through the Holy Spirit. The mystery of the incarnation consists in the assumption of a perfect humanity by the Logos. The relation of the two natures is the indwelling of the Logos in the man Jesus, as his temple or outward investiture. Through this relation the Son of David is called the Son of God, though not in the proper and exclusive sense. This view, makings, the union of the two natures an external and moral rather than substantial union, naturally led, after Nestorianism arose, to the conclusion that Diodorus and the school of Antioch had been its precursors, to say the least. See the article of Semisch in Herzog, Real-Encyklop. 1.c.; and compare Lardner, Works, 4:376 sq.; Ceillier, Histoire Generale des auteurs ecclesiastiques, 5:586 sq. (ed. of Paris, 1863-65); Gieseler, Ch. History, volume 1, § 82; Dorner, Person of Christ (Edinb. transl.), per. 2, epoch 1; chapter 1.