Diodorus (3) , presbyter of Antioch, and c. a.d. 379 bp. of Tarsus, of a noble family of Antioch, where he passed nearly the whole of his life until he became a bishop (Theod. H. E. iv. 24). He studied philosophy or secular learning at Athens, where he probably was an associate of Basil and Julian, the future emperor (Facund. lib. iv. c. 2, p. 59). On his return to his native city, Diodorus and his friend Flavian, also of noble birth (subsequently bp. of Antioch), embraced a religious life. Here, while still laymen, during the reign of Constantius, they exerted themselves energetically for the defence of the orthodox faith against the Arians, who were covertly supported by bp. Leontius, c. 350. They gathered the orthodox laity even by night around the tombs of the martyrs, to join in the antiphonal chanting of the Psalms, which, Theodoret tells us, was first instituted or revived by them, as a means of kindling religious zeal, after the model ascribed by tradition to the martyred bishop of their church, the holy Ignatius (Socr. H. E. vi. 8; Theod. H. E. ii. 24). These services strengthened the faithful to meet the persecutions. The weight of Diodorus and Flavian at Antioch was proved when in 350 their threat of withdrawal from communion induced Leontius to suspend Aetius from the diaconate (Theod. u.s. ). On the accession of Julian, his attempt to rekindle an expiring paganism provided a new field for the energies of Diodorus. With pen and tongue he denounced the folly of a return to an exploded superstition, and so called forth the scurrilous jests of Julian.
The persecution of the Catholic cause by the Arian Valens recalled Diodorus, now a presbyter, to his former championship of the Nicene faith. During the frequent banishments of Meletius, the spiritual instruction of his diocese was chiefly entrusted to him and Flavian, and Diodorus saved the barque of the church from being "submerged by the waves of misbelief" (Theod. H. E. v. 4). Valens having forbidden the Catholics to meet within the walls of cities, Diodorus gathered his congregation in the church in the old town S. of the Orontes. Immense numbers were there "fed by him with sound doctrine" (Chrys. Laus Diodori , Â§ 4, t. iii. p. 749). When forcibly driven out of this church, he gathered his congregation in the soldiers' exercising ground, or "gymnasium," and exhorted them from house to house. The texts and arguments of his discourses were chiefly furnished by Flavian, and clothed by Diodorus in a rhetorical dress. His oratory is compared by Chrysostom to "a lyre" for melody, and to "a trumpet" for the power with which, like Joshua at Jericho, he broke down the strongholds of his heretical opponents. He also held private assemblies at his own house to expound the faith and refute heresy (Theod. H. E. iv. 25; Chrys. l.c.; Facund. iv. 22). Such dauntless championship of the faith failed not to provoke persecution. His life was more than once in danger, and he was forced to seek safety in flight (Chrys. l.c. ). Once at least when driven from Antioch he joined his spiritual father Meletius in exile at Getasa in Armenia, where, in 372, he met Basil the Great (Basil, Ep. 187). The intimate terms of Diodorus and Basil are seen from the tone of Basil's correspondence.
Even more than for his undaunted defence of the catholic faith Diodorus deserves the gratitude of the church as head of the theological school at Antioch. He pursued a healthy common-sense principle of exposition of Holy Scripture, which, discarding alike allegorism and coarse literalism, sought by the help of criticism, philology, history, and other external resources, to develop the true meaning of the text, as intended by the authors (Socr. H. E. vi. 3; Soz. H. E. viii. 2; Hieron. de Vir. Illust. No. 119).
Meletius, on being restored to Antioch in 378, appointed Diodorus bp. of Tarsus and metropolitan of the then undivided province of Cilicia (Facundus, viii. 5). His career as bishop, according to Jerome (l.c. ), was less distinguished than as presbyter. He took part in the great council of Antioch a.d. 379, which failed to put an end to the Antiochene schism, as well as in the 2nd oecumenical council at Constantinople a.d. 381. By the decree of the emperor Theodosius, July 30, 381, Diodorus was named as one of the orthodox Eastern prelates, communion with whom was the test of orthodoxy (Cod. Theod. lib. xvi. tit. i. 3; t. vi. p. 9). Meletius having died during the session of the council, Diodorus, violating the compact made to heal the schism, united with Acacius of Beroea in consecrating Flavian as bp. of Antioch, for which both the consecrating prelates were excommunicated by the bishops of the West (Soz. H. E. vii. 11). As Phalerius was bp. of Tarsus at a council at Constantinople in 394, the date of Diodorus's death is approximately fixed. Facundus and others tell us that he died full of days and glory, revered by the whole church and honoured by its chief doctors, by Basil, Meletius, Theodoret, Domnus of Antioch, and even by the chief impugner of the soundness of his faith, Cyril of Alexandria.
This high credit was disturbed by the Nestorian controversies of the next cent. His rationalizing spirit had led him to use language about the Incarnation containing the principles of that heresy afterwards more fully developed by his disciple Theodorus. Thus, not without justice, he has been deemed the virtual parent of Nestorianism and called "a Nestorian before Nestorius." It was his repugnance to the errors of Apollinarianism which led him to the opposite errors of Nestorianism. His sense of the importance of the truth of Christ's manhood caused him to insist on Its distinctness from His Godhead in a manner which gradually led to Its being represented as a separate personality. He drew a distinction between Him Who according to His essence was Son of Godâ€”the eternal Logosâ€”and Him Who through divine decree and adoption became Son of God. The one was Son of God by nature, the other by grace. The son of man became Son of God because chosen to be the receptacle or temple of God the Word. It followed that Mary could not be properly termed the "mother of God," nor God the Word be strictly called the Son of David, that designation belonging, according to human descent, to the temple in which the Divine Son tabernacled. Diodorus therefore distinguished two Sons, the Son of God and the son of Mary, combined in the person of Christ. When, then, the great Nestorian controversy set in, Cyril clearly saw that, apart from the watchword θεοτόκος , which had not arisen in the days of Diodorus, what men called Nestorianism was substantially the doctrine of Diodorus as developed by Theodorus of Mopsuestia, and that Nestorianism could only be fully crushed by a condemnation of the doctrines of Diodorus as the fountain head. This condemnation was most difficult to obtain. No name was held in so much reverence throughout the East. Cyril, however, was of far too determined a spirit to hesitate. If orthodox views of the Incarnation were to be established, the authority of Diodorus must, at any cost of enmity and unpopularity, be destroyed. Every means was therefore taken to enforce, by the aid of the emperor and the patriarch Proclus, his condemnation, together with that of his still more heretical pupil Theodorus. Cyril himself, in a letter to the emperor, described them in the harshest terms as the fathers of the blasphemies of Nestorius (Theodoret, t. v. p. 854), and in a letter to John of Antioch denounced them as "going full sail, as it were, against the glory of Christ." It is not surprising that Diodorus began to be looked upon with suspicion by those who had been accustomed to regard him as a bulwark of the faith, insomuch that Theodoret, when himself accused of Nestorian leanings, did not venture to quote the words of Diodorus in his defence, though he regarded him with reverence (σέβω ), as "a holy and blessed father" (Theod. Ep. 16). In the hope of rehabilitating his credit, Theodoret wrote a treatise to prove the orthodoxy of Diodorus, which led Cyril to peruse them and to pronounce them categorically heretical (ib. Epp. 38, 52). All attempts, however, to depreciate the authority of Diodorus, both by Cyril and Rabbulas of Edessa, only exalted him in the estimation of the Nestorian party, and the opposition contributed to the formation of the independent and still existing Nestorian church, which looks upon Diodorus and Theodorus with deepest veneration as its founders. The presbyter Maris of Hardaschir, in Persia, translated the works of Diodorus into Persian, and they, together with those of Theodorus, were also translated into Armenian, Syriac, and other Oriental tongues (Neander, Ch. Hist. vol. iv. pp. 209, 284; Clark's trans. Liberat. Breviar. c. 10). Diodorus was naturally anathematized by Eutyches and his followers. Flavian III., also bp. of Antioch, was compelled by the Monophysites to pass an anathema on the writings of Diodorus and Theodorus in a.d. 499. The controversy respecting the orthodoxy of Diodorus was revived in the 6th cent. by the interminable disputes about "the Three Articles." There is a full defence of his orthodoxy by Facundus in his Defensio Trium Capitulorum " (lib. iv. c. 2). Photius asserts that Diodorus was formally condemned by the fifth oecumenical council held at Constantinople a.d. 553, but it does not appear in the acts of that council. Diodorus was a very copious author, the titles of between 20 and 30 distinct works being enumerated in various catalogues. The whole have perished, except some fragments, no less than 60 having been burnt, according to Ebed-Jesu, by the Arians. His writings were partly exegetical, mainly controversial. He wrote comments on all the books of O. and N. T., except the Ep. to the Hebrews, the Catholic Epistles (I. John however being commented on), and the Apocalypse. In these, according to Jerome (de Vir. Illust. No. 119), he imitated the line of thought of Eusebius of Emesa, but fell below him in eloquence and refinement.