The great luminary of the life of stillness, St. Isaac, was born in the early seventh century in eastern Arabia, the present day Qatar on the Persian Gulf. He became a monk at a young age, and at some time left Arabia to dwell with monks in Persia. He was consecrated Bishop of Nineveh (and is therefore sometimes called St. Isaac of Nineveh), but after five months received permission to return to solitude; he spent many years far south of Nineveh in the mountainous region of Beit Huzaye, and lastly at the Monastery of Rabban Shabur. He wrote his renowned and God-inspired Ascetical Homilies toward the end of his long life of monastic struggle, about the end of the seventh century.
The fame of his Homilies grew quickly, and about 100 years after their composition they were translated from Syriac into Greek by two monks of the Monastery of Mar Sabbas in Palestine, from which they spread throughout the monasteries of the Roman Empire and became a guide to all generations thereafter.
Though the writings of St. Isaac have been deemed fully in line with Orthodox tradition, some have proposed that his association with the Assyrian Church of the East automatically made him a Nestorian in the tradition of Theodore of Mopsuestia. Below are a few studies to help clarify this confusion:
Afterwards he [Isaac] went to the monastery of Rabban Shabur and became exceedingly well acquainted with the divine writings; at last he lost his eyesight through his reading and asceticism. He penetrated deeply into the divine mysteries and wrote books on the divine behaviour of solitaries. He said three things which were not accepted by the community. Daniel, the bishop of Bet Garmaia, was scandalized at him on account of these things which he said.
Source: From an ancient biographical text titled Ketaba de Nakfuta
Isaac belonged to the Church of the East, commonly known as ‘Nestorian’, which, however, had little to do with Nestorius, a fifth-century heretical patriarch of Constantinople. The Church of the East used the Syriac language and considered Theodore of Mopsuestia (fourth century) as its main theological and spiritual authority. The translation of Theodore’s works into Syriac was made in the fifth century and was of crucial importance for Syriac Christianity: together with his biblical exegesis, his theological views were incorporated into East Syrian tradition, including his Christological opinions. The latter became a subject of heated discussions in the Greek-speaking East after the Third Ecumenical Council (431), which condemned Nestorius. The Christology of Theodore, who made a sharp distinction between Jesus the man and the Word of God, speaking of the ‘inhabitation’ of the Word of God in Jesus as in the ‘temple’, came more and more often to be labelled as ‘Nestorian’. Finally Theodore was posthumously condemned by the Fifth Ecumenical Council (533). But for the Christians of the Church of the East he remained forever as an unquestioned authority in the field of theology. This explains the fact that the Church of the East came to be called ‘Nestorian’, the name which was never used by this Church itself as there was no historical link between it and Nestorius.
The Christology of St Isaac of Nineveh bears clear traces of Theodore’s influence. However, there is nothing heretical or unorthodox in it. Unlike many writers that belonged to the Alexandrian school of Christology and that laid emphasis on the unity of the two natures in Jesus Christ, Isaac was close to the Antiochene school, which stressed the distinction between the two natures. The most extreme exponents of the Antiochene dyophysitism (from the Greek dyo physeis, ‘two natures’) were Theodore of Mopsuestia and Nestorius, condemned as heretics in Byzantium. However, there were many moderate dyophysite writers in the fourth to seventh century, whose doctrine was considered purely orthodox. St Isaac belonged to this more moderate wing of Christological discourse, even though he used the Christological terminology deriving from Theodore for the simple reason that it was commonly used in the Church of the East to which he belonged. The following analysis of his teaching on the Incarnation of the Word and of the deification of man will demonstrate that his Christological perspective is perfectly in tune with the mainstream Christian tradition.
Source: Bishop Hilarion Alfeyev, "The Incarnation of the Word and the Deification of Man According to St Isaac of Nineveh"
A Nestorian bishop of that city in the latter half of the seventh century, being consecrated by the Nestorian Patriarch George (660-80). Originally a monk of the monastery of Bethabe in Kurdistan, he abdicated for unknown reasons after an episcopate of but five months, and retired to the monastery of Rabban Shapur, where he died at an advanced age, blind through study and austerity. Towards the end of his life he passed under a cloud as his Nestorian orthodoxy became suspected. He was author of three theses, which found but little acceptance amongst Nestorians. Daniel Bar Tubanita, Bishop of Beth Garmai (some 100 miles south-east of Mossul), took umbrage at his teaching and became his ardent opponent. The precise contents of these theses are not known, but they were of too Catholic a character to be compatible with Nestorian heresy. From an extant prayer of his, addressed to Christ it is certainly difficult to realize that its author was a Nestorian. Eager to claim so great a writer, the monophysites falsified his biography, placing his life at the beginning of the seventh century, making him a monk of the Jacobite monastery of Mar Mattai, and stating that he retired to the desert of Scete in Egypt. Since the discovery of Ishodenah's "Book of Chastity" by Chabot in 1895 the above details of Isaac's life are beyond doubt, and all earlier accounts must be corrected accordingly.
Source: Catholic Encyclopedia: Isaac the Syrian (1908)
Much has been made in some circles that St. Isaac was a member of the Church of Persia (known today as the Assyrian Church of the East), which has been associated with the Nestorian heresy. The first edition (1984) of the Orthodox English translation of St. Isaac's Ascetical Homilies contained an extensive Epilogue entitled "A Brief Historical and Theological Introduction to the Church of Persia to the End of the Seventh Century," written by Syriac scholar Dr. Dana R. Miller of Fordham University, which has been summarized thusly in the new (2011) more compact second edition:
"Saint Isaac was and still is commonly called 'Nestorian Bishop of Nineveh' and the Church of Persia of his day, 'Nestorian'. The [first edition] Epilogue endeavored to demonstrate that the teachings of Nestorius did not inform the theology of the Church of Persia; that the writings of Theodore of Mopsuestia known to her were partial and imperfect translations, and that the controversy his writings caused in the Greek-speaking world were mostly unknown to the Church of Persia, cut off by linguistic differences and political boundaries; that in some cases it was extremism on the part of the Monophysites that led the Church of Persia to take a stance that might seem to lend itself to a Nestorian interpretation, such as the cautious avoidance of the term Theotokos to avoid Monophysite Theopaschism, though she professed the Virgin's Son to be perfect God and perfect man; that the fraternal relations with Byzantium remained open: no general and hardened opposition to the Fourth [Ecumenical] Council created a final division between the Church of Persia of Saint Isaac's day and the 'Chalcedonian' Church, as it did with the Monophysites, for whom the rejection of the Council of Chalcedon became a defining element of their identity. Its aim, in a word, was to show that the Church of Persia to which Saint Isaac belonged was neither heretical in theology nor schismatic in confession." (pages 74-75, Ascetical Homilies of Saint Isaac the Syrian, Revised Second Edition, translated and published by Holy Transfiguration Monastery, Brookline, MA, 2011)
Source: OrthodoxWiki: Isaac of Syria
Prayer of St. Isaac the Syrian to Christ (in which he appeals to Christ as to one person, who is simultaneously God and man)
O Christ who are covered with light as though with garment, who for my sake stood naked in front of Pilate, clothe me with that might which You caused to overshadow the saints, whereby they conquered this world of struggle. May Your divinity, Lord, take pleasure in me, and lead me above the world to be with You. O Christ, upon whom the many-eyed cherubim are unable to look because of the glory of Your countenance, yet out of Your love You received spit upon Your face: remove the shame from my face and grant an open face before You at the time of prayer.