Since the 1980s historians of early Christianity have studied the theory and practice of asceticism with renewed interest and greater sophistication, and more recently the doctrine of the Holy Spirit has been the subject of several important essays, monographs and translations of patristic works. Therefore, it is not too surprising that someone would find a way to investigate these two themes together. By ‘ascetic pneumatology’ Thomas Humphries refers to ‘the fact that theologians applied their belief in the Holy Spirit to their ascetic systems and thereby developed particular reflections on the work of the Holy Spirit within the lives of Christians’ (p. xv). Although theologians both before and after Cassian and Gregory the Great articulated ascetic pneumatologies, Humphries sees Cassian as initiating and Gregory as bringing to a climax a particularly significant period for this theme in the Latin West. Both men highlighted the Spirit's role in reforming the soul's thoughts and desires and in forming its virtues.

It turns out, however, that there is not much ascetic pneumatology to be found between Cassian and Gregory – it seems that the dangers of Arianism, Nestorianism and Pelagianism preoccupied most of the authors of this period – and so the book is in fact better understood as an excellent study of the Holy Spirit in Latin Christian thought in the fifth and sixth centuries. That in itself is a worthy area of study, but it does mean that the book has more to interest historians of doctrine than scholars of asceticism or monasticism, who will find most relevant the chapters on Cassian, Leo the Great (who developed an ascetic pneumatology for the laity) and Gregory. Historical theologians will find, in addition to these, illuminating and intelligent discussions of, among others, Augustine, Caesarius of Arles, Faustus of Riez, Fulgentius of Ruspe, John Maxentius and the so-called ‘Scythian monks’, Prosper of Aquitaine, Vincent of Lérins, and the Council of Orange. Readers will recognise the cast of characters in a drama that historians have traditionally understood as a prolonged conflict over ‘semi-Pelagianism’. Humphries's book contributes to recent scholarship that seeks to complicate that narrative and render it less polarised. The author also avoids a narrow concentration on the doctrine of the Spirit's double procession (pp. 205–6).

Humphries helps to develop a new view of this period and geographical context in several ways. First, merely by concluding his study with Gregory the Great rather than with the Council of Orange, he shifts the focus away from a struggle over grace and free will and toward differing views of the Holy Spirit and its role in the Christian life, of which the question of predestination is only one aspect. These authors were not participating in a single conversation about the same question, but addressed a variety of issues to which the Holy Spirit was relevant. Second, he persuasively argues for the existence of multiple ‘Augustinianisms’ during this era. Not only did authors appropriate Augustine's complex and voluminous thought in different and sometimes conflicting ways, but at times they were reading different versions of Augustine's works: ignorance of, say, certain sections of his On the Trinity could lead to diverse understandings of the Spirit and of the technical terms used to speak of God as one and three (pp. 99–127). Rather than a conflict between ‘Augustinianism’ and ‘semi-Pelagianism’, we find interactions among theologians drawing on Augustine variously for diverse projects. Gregory, for example, emerges not ‘as some kind of Augustinian who attempted to be some kind of Benedictine’, but ‘as following the general tradition of Cassian, supplemented heavily by Augustine’ (p. 197).

Above all, however, Humphries renders problematic any attempt to find a single dominant narrative to characterise this period. Even the book's stated topic, ascetic pneumatology as the application of the doctrine of the Spirit to ascetic formation, fails to provide such an overarching narrative, for ‘some theologians reflected on many of the same questions [of anthropology, the Spirit, and the like] without the intent to offer formation’ (p. 206). Still, this book is so engagingly and clearly written that the reader happily follows Humphries along diverse pneumatological paths. In the conclusion, the author tries to bring unity to the authors that he has covered with the more comprehensive theme of ‘transformation’ (pp. 109–206), but his subjects remain stubbornly resistant: they continue to think ‘outside the box’ (p. 205) not only of any dogmatic creed but also of the historian's narrative. There may be no single and straightforward story to be told here, and that may be precisely what we need to learn about Christian thought in the West between Augustine and Gregory the Great.