ʿAbdīshōʿ of Nisibis (d. 1318) was a prolific author in both Arabic and Syriac. Metropolitan bishop of the ancient see of Nisibis, on the modern frontier between Syria and Turkey, he has long been famous among Syriacists for his Metrical catalogue, a poem that commemorates the authors of the East Syriac tradition and provided the basis for many modern surveys of Syriac literature. Volume iii/1 of Assemani's Bibliotheca orientalis (1725) is essentially an annotated translation of the Metrical catalogue, and this was in the turn the basis for William Wright's History of Syriac literature (1887). ʿAbdīshōʿ was also famous for his collection of East Syriac law (the Nomocanon) and his poetic collection, the Paradise of Eden, where, employing the tradition of Ephrem (d. 373) and Narsai (d. 503), he explored different themes of theology.

But ʿAbdīshōʿ's other works, many of which take the form of encyclopedias or summae theologicae, have often been neglected in modern scholarship, and his substantial Arabic oeuvre has only recently been edited. Rassi's impressive monograph is the first to treat all of ʿAbdīshōʿ's compositions together and to situate ʿAbdīshōʿ in the intellectual currents of his day, both Christian and Muslim. Rassi refuses to reduce ʿAbdīshōʿ to a waypoint in the sterile debate over whether we should see the thirteenth century as a period of decline or renaissance for Syriac culture. He argues that we should read ʿAbdīshōʿ's writings as productive works in their own right, rather than studying them primarily for the information they give about a pre-Islamic golden age.

In chapter i, Rassi frames ʿAbdīshōʿ at the meeting point of two cosmopolitanisms, of a Syriac culture that, in the Mongol period, stretched into the Far East and transcended different, competing Christian confessions (‘Melkites’, ‘Jacobites’ and ‘Nestorians’, in the language of the Arabic sources) and an Arabic culture that was shared by Muslims, Jews and Christians and was the language of the Quran, but also of science and political discourse in the Middle East. Using the terminology of Marshall Hodgson, Rassi argues that ʿAbdīshōʿ was an Islamicate thinker, ‘a participant in a lettered tradition … naturally shared in by both Muslims and non-Muslims’ (p. 40). ʿAbdīshōʿ was not simply using Arabic for instrumental ends as a widespread language. Instead he wrote for Christian audiences who were fully conversant with Arabic norms and used them on their own terms. He drew on a Christian Arabic apologetic tradition that had been flourishing since the ninth century, but he also employed turns of phrase drawn from the Quran that might appear Muslim to European readers but which had become Arabophone commonplaces.

ʿAbdīshōʿ drew on summae theologicae from several different Christian traditions, as well as using epitomes of church history, church law and systematic theology. This tendency led an earlier generation of scholars to condemn him as unoriginal (p. 29), but Rassi reminds us that, though ʿAbdīshōʿ humbly denies his own personal contribution, his use of the past was highly selective (p. 48). His aim was to simplify for teaching purposes, as well as to expand and develop earlier ideas in response to new threats or new modes of expression. Thus he ‘[upheld] a stable and circumscribed body of dogma for Christians living in an increasingly non-Christian environment’ (p. 49). Rassi's point is one that should hold true for many cultures that have preferred to develop their ideas in a tradition of commentary or summary, rather than through the celebration of novelty.

Chapter ii sets out the slim pickings of what we can know of ʿAbdīshōʿ's career and his political context in the Mongol Ilkhanate, when hopes for a Christian Mongol ruler came to be replaced by bouts of Muslim-Christian violence. Chapter iii deals with ʿAbdīshōʿ's defence of the Trinity across his works. Rassi analyses how he drew on ninth-century apologetic writings that had adapted Aristotelian logic and the Greek and Syriac patristics to a Muslim context as well as responding to the later development of Aristotelian thought by Muslim thinkers such as Avicenna (d. 1037).

Chapter iv investigates ʿAbdīshōʿ's defence of the two-nature Christology of the Church of the East. This Christology had been formed by strife between different Christian confessions in the sixth and seventh century, but Rassi observes how these debates had then provided the raw materials for Christian thinkers to defend the incarnation before Muslim interlocutors. Though ʿAbdīshōʿ never departs from the Dyophysite theology of his own Church, he also shows himself willing to use the ideas of non-Dyophysites at many points, and, as he grew older, he increasingly distanced himself from divisive language when talking about other Christian confessions and downplayed the differences between their Christologies.

The discussion of how the language of Muslim theologians was incorporated into ʿAbdīshōʿ's defence of the incarnation was particularly fascinating. While Christian theologians had developed an established vocabulary of analogies to describe the incarnation, ʿAbdīshōʿ uses the novel example of translucent objects that are placed together, such as wine and a wine glass, being analogous to Christ's humanity and his divinity (p. 169). This image seems to come from Sufi thought, where the wine and the wine glass stand for the unification of intent between God and a mystic. ʿAbdīshōʿ deliberately adapts this simile to make the Sufi image a way of understanding the incarnation. Perhaps he felt that non-Christians were capable of mystical insights that would allow them to understand the incarnation if they became aware of the deeper significance of their ideas.

Chapter v defends the Christian veneration of the cross and their use of wooden clappers to bring the faithful to church. Muslims accused Christians of innovating new practices that had no basis in the Bible, and Ibn Taymiyya (d. 1328) saw the veneration of the cross in particular as shirk, the sinful association of other beings or objects with God (p. 204). ʿAbdīshōʿ defends the cross as rooted in Scripture, as a type that is attested in the Old Testament (the staff of Moses in Numbers xxi.8–9) and as the means by which Christ's redemption of mankind was enacted, and in church history, as the source of the Apostles’ miracles and in the miraculous discovery of the cross by Helena (pp. 209, 215). Much of this defence re-uses ideas that had been current in apologetics since the ninth century, but Rassi draws out how ʿAbdīshōʿ's terminology reflects ongoing Muslim debates. For instance, he describes the redemption effected by the cross as ‘divine aid’ (lutf). This expression had acquired a central place in Mutazilite theology, where God's assistance to mankind in preventing harm is a necessary consequence of God's existence (pp. 175, 211).

This is a masterful study that blazes a trail for others to follow. Grounded in a very rich range of sources, Rassi is fully in command of the intertexts that ʿAbdīshōʿ evokes or redeploys, from Greek and Syriac patristics and the earliest Christian Arabic apologetics to a range of Muslim interlocutors. He makes a good case for the importance of studying this period on its own terms, and the usefulness of reading Muslim and Christian material to illustrate one another, especially when points of contact may not be directly acknowledged by the authors themselves. Above all he argues that it makes little sense to segregate world Christianities by language and culture: a figure like ʿAbdīshōʿ only makes sense as a man fully at home in both Syriac and Arabic.