Much has been made in some circles that St. Isaac was a member of the Church of Persia (known today at the Assyrian Church of the East), which later became connected in certain ways with the Nestorian heresy. The first edition of the Orthodox English translation of the 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," which has been summarized thusly in the new, 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 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."
St. Isaac is fully accepted as a saint in the Orthodox Church, though during his lifetime, he was [[canons (law)|canonically]] a member of the [[Assyrian Church of the East|Church of the East]], a church that has been associated with the [[Nestorianism|Nestorian heresy]], although that charge is contested today. His writings nevertheless came to be extremely popular in Orthodox monastic circles and are well-known for their Orthodoxy. Most contemporary Nestorians rejected St. Isaac's three theses, which, although they are not known in their exact form, were Orthodox and incompatible with the Nestorian heresy.[http://www.newadvent.org/cathen/08176a.htm] One of his extant prayers to Christ makes it difficult for one to maintain that the holy monk was a Nestorian.[http://www.newadvent.org/cathen/08176a.htm] [[Veneration]] for him grew, and he came to be incorporated into the Orthodox calendar of saints.