At Edessa, capital of the principality of Osrhoëne (in eastern Syria), and western Mesopotamia neither Latin nor Greek was understood. Therefore, the native language Syriac (a Semitic language related to Aramaic) was used in Christian writings. The political fortunes of Edessa present a remarkable contrast to those of other centers of Christianity. Until 216 CE in the reign of the Emperor Caracalla, Edessa lay outside the Roman Empire. Christianity seems to have reached the Euphrates valley about the middle of the 2nd century, that is, while the country was still an independent state. Since its people did not speak Greek, like their neighboring Syrians in Antioch, it is not surprising that the Christianity of Edessa began to develop independently, without the admixture of Greek philosophy and Roman methods of government that at an early date modified primitive Christianity in the West and transformed it into the amalgam known as Catholicism.
According to early traditions and legends embodied in the Doctrine of Addai (~400 CE), the earliest New Testament of the Syriac speaking Church consisted of the Diatesseron, the Epistles of Paul, and Acts. The Diatesseron was written by Tatian by weaving the 4 canonical Gospels together into a coherent and continuous account. Tatian was born of pagan parents in the land of the Assyrians and received an education in Greek culture and its philosophical systems. Tatian came to Rome, made the acquaintance of Justin Martyr, and converted to Christianity. While there, he composed the Diatesseron about 150 CE. The original language of the Diatesseron (certainly either Old Syriac or Greek) is still a much-debated question. The term diatesseron borrowed from musical terminology and designated a series of 4 harmonic tones. It was Tatian’s private judgment that the format of a fourfold harmony was the most convenient way in which to present the whole Gospel story at once instead of confusing people by offering them 4 parallel and more or less divergent narratives.
After Justin’s martyrdom (~165 CE) Tatian broke with the Roman church, returned to Syria in 172, and founded the sect of the Encratites (i.e. the self-disciplined). This sect rejected matrimony as adultery, condemned the use of meat in any form, and substituted water for wine in the Eucharist service. While in the East Tatian introduced the Diatesseron among the local churches. His influence at Edessa must have been considerable, for he succeeded in getting his book read in the churches there, and afterwards its use spread throughout the region. It was quoted by Aphraat, Ephraem (who wrote a commentary on it), and other Syrian Fathers.
Because of Tatian’s reputation as a heretic, however, a reaction set in against the use of his Diatesseron, and Bishop Rabbula of Edessa (d. 436 CE) instructed his priests to take care that in all the churches the 4 ‘separated’ Gospels should be available and read. Theodoret, who became bishop of Cyrrhus on the Euphrates in upper Syria in 423, sought out and found more than 200 copies of the Diatesseron, which he ‘collected and put away, and introduced instead of them the Gospels of the four evangelists’.
By the beginning of the 5th century, or slightly earlier, the Syrian Church’s version of the Bible, the Peshitta (‘simple’ translation) was formed. For the New Testament it represented an accommodation of the Syrian canon with that of the Greeks. It contains 22 books – all of the present New Testament except:
II Peter, II John, III John, Jude, Revelation of John
For the eastern part of the Syrian Church this constituted the closing of the canon, for after the Council of Ephesus (431 CE) the East Syrians separated themselves as the Nestorians. There are many surviving manuscripts of the Peshitta, the oldest of which bears the date 442. For much more on Peshitta history, see the article at The Encyclopedia of New Testament Textual Criticism. It is noteworthy that exactly these 22 books are cited by John Chrystosom (~347-407) and Thedoret (393-466) from the School of Antioch. For a visual summary of these 22 books see the Cross Reference Table.
Among the Western Syrians, however, there were closer ties with their neighboring Churches, and a further accommodation with the Roman church took place in the 6th-7th centuries when the Philoxenian and Harclean versions of the Peshitta were issued containing all 27 New Testament books. Yet, even so the West Syrian Church was slow in making use of these parts of the New Testament.
Still today the official lectionaries followed by the Malankara Syrian Orthodox Church, with headquarters at Kottayam (India); and the Chaldean Syrian Church, also known as the Church of the East (Nestorian), with headquarters at Trichur (India); present lessons from only the 22 books of the original Peshitta.