DIODORUS. - Bishop of Tarsus, ascetic and second founder of the Antiochene exegetical school, b. in the first quarter of the 4th century, d. before 392. He studied at Athens (Facundus of Hermiane, Pro defensione trium capit., 4, 2: PL 67, 621 B, 11-14), acquiring a vast secular culture, so that St. Jerome's observation attributing to him "ignorantiam saecularium litterarum" (De vir. illustr., 119: PL 23, 750) is not accurate. He was also a disciple of Silvanus, bishop of Tarsus, and of Eusebius of Emesa.
After his schooling D. devoted himself to asceticism in an asceterion near Antioch: there he applied himself to the study of Holy Scripture, and became, together with his companion Carterius, the superior of a group of ascetics among whom were St. John Chrysostom and Theodore of Mopsuestia.
Under the Arian emperors Constantius (337-61) and Valens (364-78) D. and his friend Flavian, not yet priests, carried on tireless activity in defense of the faith of Nicaea. When Leontius, Arian bishop of Antioch (cf. Theodoret, Hist. eccl., 2, 19: PG 82, 1060), placed the Arian Aetius among the deacons, Flavian and D. protested and Leontius yielded. In those same times he taught the people to sing psalms and hymns antiphonally in the assemblies of the orthodox outside the Arian churches. This singing met with such success that Leontius himself requested that these gatherings be held in the churches (Theodoret, ibid.); Semitic sources report that D. had learned this custom during his journey to Persia.
In 361 an orthodox man, St. Meletius, having become bishop of Antioch, ordained the two friends as priests. When Julian the Apostate passed through Antioch the following year, D. was a priest and known to the emperor as the leader of the Christians. Around 372 he went to Armenia, where his bishop Meletius was living in exile; there he also found St. Basil, with whom he formed bonds of friendship, as appears from the Basilian correspondence (Epist., 135 and 244, 3: PG 33, 572 and 916). On the death of Valens (378) he was consecrated bishop of Tarsus in Cilicia. He took part in the Antiochene Council of the following year and in that of Constantinople of 381 (Theodoret, Hist. eccl., 5, 4: PG 82, 1204 A). In a decree of 30 July 381 Theodosius proclaimed as models of the orthodox faith, for the diocese of the East, Pelagius of Laodicea and D. of Tarsus (Hefele-Leclercq, II, 41). From St. Jerome's notice of the year 392 (De vir. ill., 119) it appears that D. was already dead.
Of D.'s numerous writings (the Suda has transmitted a list of them to us: PG 33, 1553) a great part has been lost. D. commented on almost all the books of the Old and New Testament. There remains from him a treatise on hermeneutics with the suggestive title: What is the difference between theory and allegory? The polemical-dogmatic treatises were for the most part directed against the errors of the Jews, pagans, Arians and other heretics. Besides the works listed by the Suda, Photius makes known to us a book against the Manichaeans and one on the Holy Spirit (Photius, Bibl., cod. 85, 102: PG 103, 288B, 372); St. Jerome knew a commentary on the letters of St. Paul; and Leontius of Byzantium a work Contra synousiastas, by this name meaning the Apollinarists. For the Syriac lists v. the article of R. Abramovski, Untersuchungen zu D. von Tarsus, in Zeitschr. f. Neutest. Wissensch., 30 (1931), p. 245 ff.
The fragments that have come down to us are found either in exegetical catenae or in polemical florilegia, whence the difficulty of establishing their authenticity and of understanding the author's thought through passages torn from their context.
Migne collected some fragments on the Octateuch and the Psalms (PG 33, 1561-1628), later increased in Deconinck's edition (further completion by R. Devreese, in Rev. bibl., 45 [1936], pp. 217-20). The hypothesis of Mariès attributing to D. the complete commentary on the Psalms of Anastasius of Nicaea is not confirmed. The passages on the Epistle to the Romans have been edited by Staab.
There remain the few dogmatic-polemical passages. Those collected in PG 33, 1560 ff. are taken either from Leontius of Byzantium (Contra Nestorianos et Eunomianos, III, 43: PG 86, 1, 1385 ff.) or from Cyril of Alexandria (fragments of the work Contra Diodorum et Theodorum: PG 76, 1437 ff., some in Mansi, IX, 215 b, 219 b c, 231: Council of Constantinople, III, 553). Other fragments exist in the Palatine Collection of the Council of Ephesus (E. Schwartz, Acta conc. oec., I, 5, pp. 177-79, earlier than the year 550); Syriac ones in M. Brière, Fragments syriaques de D. d. T. réédités et traduits, in Revue de l'Orient chrétien, 3rd series, 10 (1946), pp. 231-85. Abramovski has studied the tradition of these fragments in the manuscripts (ibid., pp. 247-53) without touching the problem of their authenticity. M. Richard (Les traités de Cyrille d'Alexandrie...), studying the aforesaid fragments, arrives at the following conclusions: D. wrote a work Contra synousiastas (that is, against the Apollinarists) in two books. Against the first book St. Cyril polemicizes in his Contra Diodorum. Six fragments of this first Diodorean book are transmitted to us in the Palatine Collection (the fourth fragment, homo ungitur Iesus, is by Paul of Samosata); seven or more others may be found in the Philalethes and in the Contra impium grammaticum of Severus of Antioch. Against the second Diodorean book St. Cyril likewise polemicized, attributing it to Theodore. Cyril, when he wrote, had before his eyes not the works themselves of D. and Theodore but florilegia compiled by the Apollinarists (and therefore probably also falsified). In this compilation, offered to him at Jerusalem in 438, Cyril would have found already existing the confusion between D. and Theodore, which, writing in haste, he did not notice.
There remains furthermore the long analysis of Photius (Bibliot., 223: PG 103, 829-77) of the fragments of the Contra fatum, which have been studied by V. Ermoni (Diodore de Tarse et son rôle doctrinale, in Le Muséon, 2 [1901], pp. 424-44) and by P. Doll (Κατὰ εὐμερίενης, Bonn 1923).
For a stylistic judgment on D., given the lack of complete works, one must be content with the criticism of Photius (Bibliot., 223: PG 103, 820 and 877) and of St. Basil (ep. 135: PG 33, 572).
Given the uncertainty of the sources, D.'s doctrine cannot be set forth. Basing ourselves however on the good Antiochene doctrine in the introduction to the Psalms published by L. Mariès, the following is noted. The author is concerned for the understanding of the Psalms sung by the brethren (p. 82, lines 12 f., 24 f. and p. 86, 9 f.); he struggles against two opposite excesses: Jewish literalism and allegorism of a pagan stamp; he wishes to follow the middle way of "theory" (p. 88, 22 f.), that is, the comprehension of the prophetic sense regarding Christ, while however always presupposing the historical sense (92, 4 f. and 88, 3 f.). When he speaks of allegory, he refers sometimes to Christians who by their allegorism do injury to the sense of Holy Scripture (p. 88, 11), sometimes also to pagans who hold the notion that allegory (p. 90, 6 f.) completely removes the historical sense. This attention to the historical sense does not even prevent Holy Scripture from being a reality adaptable to the needs of many by reason of the Grace of Him who works within (p. 96, 5 f., and especially 98, 19 f.). Cf. A. Vaccari, La « teoria » esegetica antiochena, in Biblica, 1 (1920), pp. 12, 13, on the Antiochene theory, and for the relation between the exegesis of the Antiochene and Alexandrian schools cf. also H. de Lubac, Typologie et allégorisme, in Recherches de science, religieuse, 34 (1947), pp. 180-226.
For the dogmatic and especially christological doctrine of D., one must note the Aristotelian influence not only in certain points of his physical doctrine but also in his manner of reasoning (cf. Ermoni, op. cit., p. 433 ff., and Abramowski, op. cit., p. 254). According to a Syriac fragment cited by Abramowski (ibid.), D. would have placed reason alongside the Bible as a source of theology. Writing against Apollinaris of Laodicea, D. instead taught the perfect humanity and the divinity of Christ, separating the divine notions from the human ones, separating the Son of God from the Son of David, God the Word from the man Jesus Christ. The Son of God is not the son of David but rather his Lord (PG 33, 1560C). The union of the two sons is explained as an indwelling of God the Word in his temple Jesus (PG 33, 1560AB) or under the image of a garment. The Word would have clothed himself with the humanity of Jesus (Schwartz, op. cit., p. 179, line 4). It must be remembered, however, that these observations are based exclusively on fragments selected after D.'s death in the anti-Nestorian struggle to show that Nestorius' error already had its precedents. St. John Chrysostom, his great disciple, has not left us even a hint about his master's doctrine.