From 423 to 457, Theodoret was bishop of Cyrrhus, a small town in Syria two days' journey from Antioch. In his Haereticarum fabularum compendium — commonly known as his History of Heresies — he mentions the Diatessaron:
This one [Tatian] also composed the gospel called Diatessaron by cutting by cutting out the genealogies and whatever goes to prove the Lord to have been born of the seed of David according to the flesh. And this work was in use not only among his own party but also among those who follow the Apostolic teachings, who used it somewhat too innocently as a compendium of the Gospels, because they did not recognise the wickedness of its composition. I myself found more than two hundred copies in reverential use in the churches of our diocese, and all of them I collected and removed and instead of them I introduced the Gospels of the four Evangelists. - haer. fab. comp., I.2018
Theodoret, like the Latin and Syriac translations of Eusebius, and like Epiphanius, calls the Diatessaron a "gospel." Theodoret is the first writer to assert that the Diatessaron excised the genealogies. Subsequent discoveries of Diatessaronic witnesses have shown his statement to be accurate. He also states that the Diatessaron was used by orthodox Christians. Elsewhere, Theodoret informs us that his diocese consisted of eight hundred Syriac-speaking parishes. The fact that he confiscated "more than two hundred copies" of the Diatessaron and "introduced the Gospels of the four," means that over twenty-five per cent of the parishes in his diocese not only used the Diatessaron, but also lacked the separate gospels as late as the early fifth century.
Corroboration for Theodoret's report comes from one of his contemporaries Rabbula, bishop of Edessa from c. 412 to 435, was a rigorous enforcer of orthodoxy who promulgated a series of canons. Canon 43 stipulates the use of the fourfold or "Separate" gospels.
The priests and deacons should exercise [due] care that in all the churches a copy of the Euangelion da-Mepharreshe [the Gospel of the Separated] shall be present and shall be read.
Although the Diatessaron is not named, it was "evident that when Rabbula became bishop of Edessa the form in which the Gospel was practically known to Syriac-speaking Christians was Tatian's Harmony."(Euangelion da-Mepharreshe, ed. Burkitt II.154)
There are, however, earlier and other references to Tatian — in Irenaeus,24 for example. None, however, mentions the Diatessaron. Clement of Alexandria names two of Tatian's treatises by title, but not the Diatessaron; his silence is even more significant if, as has been suggested, he were a student of Tatian.
Another odd fact is that — with three exceptions — all of these early notices of the Diatessaron come from Greek authors in the Eastern provinces of the Roman Empire: Eusebius in Palestine and Epiphanius on Cyprus. The exceptions are the Syriac Doctrina Addai, and the Syriac and Latin translations of Eusebius' h.e. Apart from Rufinus' reference, the first European or African writer to mention the work (and he calls it a diapente, not a diatessaron) is Victor of Capua, in his preface to the Latin Codex Fuldensis, dated 546 (see infra, item E). The obvious conclusion — which may or may not be valid, since it is e silentio — is that these earliest (and Western?) Fathers were ignorant of the Diatessaron and, consequently, its attribution to Tatian. The silence of these Fathers will be considered when we deal with the question of the provenance of the Diatessaron.
Thus far (with the exceptions of the Doctrina Addai and the Syriac translation of Eusebius) we have focused on evidence from the Greek Fathers, for they offer the earliest testimony concerning Tatian and the Diatessaron; but the first citations from the Diatessaron come from the Syrian East. Ephrem Syrus (d. 373) wrote a Commentary on the Diatessaron. It survives in three manuscripts, one in Syriac, which lacks the first folio with the title, and two in Armenian. The Armenian title is, following the usual translation, "Commentary on the concordant gospel." The Armenian word ( ) can also mean "symphonic" or "homophonic." On the basis of a Syriac catena (Vatican: MS Borgia Syriaca 82; the catena attributes a quotation to "Mar Ephrem the Teacher, from the Commentary of the Diatessaron (Diastarun sic!) Gospel." Tj. Baarda conjectured that the Syriac title of Ephrem's work was "Commentary of [or: on] the Diatessaron Gospel."30 If Baarda is correct, then Ephrem's title would be the second use of the word "Diatessaron" in Syriac, the first being in the Syriac translation of Eusebius' h.e. That Ephrem's Commentary was written on the Diatessaron is certain, for its sequence of pericopes and variant readings follow those of the Arabic Harmony quite closely.
In addition to the Commentary, Ephrem's sermons (both prose and metrical) and hymns frequently quote the gospels in the form of the Diatessaron, not the separate gospels. Similarly, the gospel citations of Apharat, the "Persian sage," (d. 350?) a slightly older contemporary of Ephrem, also frequently agree with the Diatessaron. Aphrahat provides an interesting bit of information which, as it turns out, is relevant to Diatessaronic studied. In his Demonstrations I.10 one reads:
as it is written in the beginning of the Gospel of our Saviour, "In the Beginning was the Word."Later Syrian writers (see infra, items N and Q) reiterate that the Diatessaron began with John 1.1; and indeed, many existent Diatessaronic witnesses commence with John. This report of Aphrahat's, then, is usually understood as referencing the Diatessaron — which he calls "the Gospel of our Saviour." It is the earliest description of a physical feature of the Diatessaron; Theodoret's negative statement, reporting the lack of genealogies, is the second.[Petersen, Diatessaron p. 44 - 45]
