On his return from Alexandria he was appointed bishop of Edessa by his friend Athanasius II, Patriarch of Antioch. He held this office for three or four years, as the clergy opposed his strict enforcement of the Church canons and he was not supported by Julian, the successor of Athanasius. Publicly burning a copy of the canons in front of Julian's residence, Jacob retired to the monastery of Kaisum near Samosata, and from there to the monastery of Eusebhong where, for eleven years, he taught the Psalms and the reading of the Scriptures in Greek. Towards the close of this period Jacob again encountered opposition, this time from monks who despised the Greeks.
Jacob left Eusebhong and proceeded to the great convent of Tell Add or Teleda (? modern Tellgdi, northwest of Aleppo), where he spent nine years in revising and amending the Peshitta version of the Old Testament with the aid of the various Greek versions.
He was finally recalled to the bishopric of Edessa in 708, but died four months later.
Jacob belong to the Syriac Orthodox Church and his writings have a definite Monophysite character. Assemani tried hard to prove him orthodox (B.O. i. 470 sqq.) but changed his opinion on reading his biography by Barhebraeus (ib. ii. 3-7). See especially Lamy, Dissert. de Syrorum fide, pp.206 sqq. i Text at Leipzig 1889 (Des Buch der Erkenntniss der Wahrheit oder der Ursache alter Ursachen): translation (posthumously) at Strassburg 1893.
Most of his works are in prose. Few have been published. In 1911 most of the information available was to be found in Giuseppe Simone Assemani's Bibliotheca Orientalis and Wright's Catalogue of Syriac MSS in the British Museum.
Jacob produced a revision of the bible, based on the Peshitta. Wright calls this a curious eclectic or patchwork text. Five volumes survive in Europe (Wright, Catalogue 38). This was the last attempt at a revision of the Old Testament in the Syriac Orthodox Church. Jacob was also the chief founder of the Syriac Massorah among the Monophysites, which produced such manuscripts as the one (Vat. cliii.) described by Wiseman in Horae syriacae, part iii.
He also wrote commentaries and scholia on the bible. Specimens of these are given by Assemani and Wright. They are quoted a lot by later commentators, who often refer to Jacob as the "Interpreter of the Scriptures".
He also wrote a Hexahemeron, or treatise on the six days of creation. Manuscripts of this exist at Leiden and at Lyon. It was his last work, and being left incomplete was finished by his friend George the bishop of the Arabs.
Jacob made a collection of ecclesiastical canons. In his letter to the priest Addai we possess a collection of canons from his pen, given in the form of answers to Addai's questions. These were edited by Lagarde in Reliquiae juris eccl. syriace, pp. 117 sqq. and Jean Baptiste Lamy in Dissert. pp. 98 sqq.
Additional canons were given in Wright's Notulae syriacae.
All of them have been translated and expounded by Kayser, Die Canones Jacobs von Edessa (Leipzig, 1886).
He also made many contributions to Syriac liturgy, both original and as translations from Greek (Wright, Short History, p. 145 seq.).
Jacob's chief original contribution was his Enchiridion or Manual, a tract on philosophical terms (Wright, Catalogue 984).
Translations of works of Aristotle have been attributed to him. However these are probably by other hands (Wright, Short History p. 149; Duval, Littirature syriaque, pp. 255, 258).
The treatise De cause omnium causarum, which was the work of a bishop of Edessa, was formerly attributed to Jacob; but the publication of the whole by Kayser has made it clear that the treatise is of much later date.
The original text of Jacob's continuation has unfortunately perished apart from a few leaves in a manuscript in the British Library. Of these a full account is given in Wright, Catalogue 1062, and an edition of these has been published in CSCO by E.W.Brooks.
Jacob is most famous because of his contributions to the Syriac language and the West Syriac script (Serto).
In early Syriac vowels are either not shown, or else shown by a system of dots, used erratically. Jacob borrowed five vowel signs from Greek, which he wrote above the line as miniature symbols. This caught on, and is a feature of West Syriac writing even today. He also elaborated the system whereby some consonants could stand for vowels.
Jacob also tried to introduce the Greek practice of writing the vowels on the line, the same way as is done for the consonants. This was resisted by his countrymen and did not come into effect.
In his letter to George, bishop of Shrugh, on Syriac orthography (published by Phillips in London 1869, and by Martin in Paris the same year) he sets forth the importance of fidelity by scribes in the copying of minutiae of spelling.
As a translator Jacob's greatest achievement was his Syriac version of the Homiliae cathedrales of Severus, the monophysite patriarch of Antioch. This important collection is now in part known to us by E. W. Brooks's edition and translation of the 6th book of selected epistles of Severus, according to another Syriac version made by Athanasius of Nisibis in 669. (Pseudo-Dionysius of Tell-Mahr says 677; but Athanasius was patriarch only 684-687.)
A large number of letters by Jacob to various correspondents have been found in various MSS. Besides those on the canon law to Addai, and on grammar to George of Serugh referred to above, there are others dealing with doctrine, liturgy, and so forth. A few are in verse.