Marijke Metselaar's Defining Christ: The Church of the East and Nascent Islam offers an extensive, detailed exposition of the development of the Christological doctrine of the Church of the East in late antiquity. Though the book's narrative begins much earlier, its focus is the seventh century, a crucial period that saw a series of definitive theological debates within the Church of the East as well as the establishment of the Islamic caliphate as the dominant power in the church's heartlands of Iraq, Syria, and Iran.

The Church of the East, known also as the East Syrian and sometimes the “Nestorian” Church, denotes the communion of Christian bishoprics, monasteries, and laypeople in the Persian Sasanian Empire of late antiquity—that is, the church east of Roman territory, which used Syriac as its principal literary and liturgical language. Metselaar begins in the beginning, providing in chapter 1 an overview of the Church of the East's origins and its major theologians from the fourth century to the opening of the seventh. She introduces those theologians’ usage of the key Syriac technical terms—parṣopa, qnoma, and kyana or ousiya—that render the concepts of Christ's person, hypostasis, and nature so central to the divisive Christological controversies of the fifth century. Chapter 2 surveys the main figures of the Church of the East in the first half of the seventh century, the period in which Babi the Great propounded what would soon become the church's definitive Christological doctrine of two natures, one person, and two hypostases.

Chapters 3 and 4, the book's heart, focus on Catholicos Ishoʿyahb III, chief bishop of the Church of the East from 649 to 659 in the early years of the Islamic caliphate. The interest here lies in Ishoʿyahb's corpus of 106 Syriac letters, which represent a far larger amount of epistolary material than what is attributable to almost any other premodern Syriac bishop and thus a promising source base on which to examine Ishoʿyahb's mid-seventh-century world. Chapter 3 analyzes the letters to consider Ishoʿyahb's life in political and communal leadership, focusing on his dealings with Miaphysites within the Church of the East, ecclesiastical opponents, and the new Arab ruling administration. Chapter 4 resumes Defining Christ's main narrative by turning to Ishoʿyahb's Christological and other theological writings. A shorter chapter 5 closes the book by bringing the survey of Church of the East Christological writings down to 700 (an end date that appears to be arbitrary; other than marking the death of a sitting catholicos, it does not correspond particularly meaningfully to events in the surrounding caliphate).

Defining Christ does a great service in the detail and thoroughness with which it examines the development of the Church of the East's Christological doctrine. It remains common enough to read in both general and specialist works simplistic summaries of the major Christological positions of late antique Christian traditions—Chalcedonian, Miaphysite, East Syrian, and others—as if these had been conclusively formulated at singular moments in time. Metselaar musters a considerable volume of both primary source texts and scholarship to chart the multiple understandings of those crucial, contested terms that, even if the underlying Christological concepts remained largely consistent within a given tradition, were nonetheless employed differently by different thinkers, both before and after their most definitive formulations. Metselaar's attention to the often very different employment of multivalent terms like qnoma across different literary genres is particularly welcome.

The book's interest in placing “Christological questions . . . in the context of the main political events of the time” (4) does not always bear as much fruit, especially with regard to the nascent Islam of the book's title. The letters of Ishoʿyahb are the main tool at the author's disposal for pursuing this line of inquiry, but their highly rhetorical style and their relatively meager allusions to the ruling power of the seventh-century Middle East make their interpretation difficult. As a result, connections drawn between Islam and East Syrian Christology or Ishoʿyahb's letters are sometimes tenuous and more speculative than substantiated. For example, the suggestion that Ishoʿyahb may have avoided particular epithets for Jesus, like Son of God, that run counter to Islamic Christology seems a stretch (337–345); robustly Christian writing has always persisted in Islamic polities and Muslim-majority societies, and there is no evidence of which I am aware to indicate the existence of a state censor of Syriac letters that Ishoʿyahb had to fear. To its credit, the book offers this and other suggestions like it tentatively, but perhaps a sounder analytical strategy would have been to focus only on the most likely allusions to Islam and Arab rulers and to leave aside entirely the tenuous, unconvincing ones.

A stronger editorial hand might also have burnished the book's organization. The first four of the book's five chapters are quite long, between 83 and 114 pages each, and they are more expository and descriptive than argumentative. There is no denying the richness and complexity of the material that the book explicates; tighter, more coherently argumentative chapters might have helped Defining Christ's narrative presentation of that material.

All monographs have their shortcomings, and, these aside, Defining Christ is an admirably thorough examination of East Syrian theology over the course of a critical century. The degree of detail it offers across a range of East Syrian theologians will make it useful even as a reference work, and it should be of great interest to advanced graduate students and scholars of theology and Christian late antiquity.