One advantage of the Christian narrative of triumph over a multiplicity of demons is its simplicity. The ‘one’ is superior to the many, the simple to the complex (whether or not the formalisation of Trinitarian doctrine undercuts this is another matter). God and his Word appear to offer more clarity than the intricacies of innumerable divine entities embedded in literature, domestic life, civic observance, military operations. This book is not a study of the role of demons in early Christian history per se. It restricts its scope to the functions that they perform in Eusebius’ writings. As such it is not concerned directly with either scriptural witnesses to hierarchies of more-than-human beings whether evil or good (angels, demons), or to anthropological and sociological understandings of the role of (for want of a better term) ‘spirits’ as agents of influence in human existence. One of the most important questions it raises, and flags up in conclusion as a subject requiring further research, is that of how demons relate to human agency, what historians would think of in terms of causation, either individual or collective (for demons can appear in either form). This is a question familiar from Classical literature in general, and tragedy and epic in particular, in which the layers of causation created by human and divine agency create for the reader a ‘double-perspective’, and often dramatic irony as well. The author encourages readers to engage with questions about reality and physicality in Eusebius’ writings, and the nexus of multiple meanings in which the literal has to be teased out from the metaphorical – and this when the very semantic range of such key words as ‘demon’ is what precludes choosing firmly between physical reality and moral trope.

Even the fact of this book's having been written shows that important insights have been gained in recent decades. Demons and demonology are not folk religion, or primitive religion, or religion for the poor or stupid in contrast with religion for the rich, educated and intellectual. Johannessen emphasises the role and importance of demons in Christian writers of the fourth century who are commonly considered as far more sophisticated than Eusebius (such as the Cappadocians in the East; or, for that matter, Augustine in the West) as having been misleadingly underplayed. Perhaps an analogy with the prodigy-lists in Livy, once similarly sidelined as peripheral folk-religion, is not inappropriate here.

There are many positives to the author's analysis beyond the larger point about restoring demons to their proper place within the thought-world of late antique religion. She gives a clear, concise impression of Eusebius’ oeuvre; she sensitises the reader to terminological anachronisms like ‘free will’. ‘political philosophy’ and ‘moral responsibility’. She is not afraid to tackle questions of dating, or the degree of acquaintance exisiting between Constantine and Eusebius, which have come to the fore through the work of T. D. Barnes, especially in his Constantine and Eusebius. A careful path is picked out between approaches which treat demons as mere metaphors, and those which invest them with interior will and self-awareness. In this she make a case, not altogether convincingly, for Eusebius having had a hand in negativising the demonic, shifting away from a more neutral approach. Although she touches on questions of genre, particularly apologetic, this is not the most convincing part of the argument, perhaps because it is too tempting to emphasise the importance of Eusebius’ writings in terms of both content and influence. It has to be said that the brilliance of the proem to Historia ecclesiastica (in both ideas and expression) is not matched by anything in the subsequent content of the work. It is true that Eusebius is neither entirely coherent nor systematic in his treatment of demons, and Johannessen has to work hard to make a case for his writings yielding anything like a consistent political or ethical approach. She detects subtle advice and direction to rulers, where other scholars see much less – demons can be a trope for explaining change from good to bad (which is always potentially tricky in ancient interpretations of character), and a way of demarcating moral excellence from mere good fortune, without diminution of their ontological reality.

I am not convinced that Barnes's depiction of the emperor as believing that he had a special mission to establish Christianity is best summarised in terms of his being a ‘committed Christian’ (p. 3); surely here, if anywhere, is an anachronisic thought-category. The least convincing part of the book came with the argument that Eusebius used demons as a way of identifying a tyrant: it is much more likely that the tyrannical nature of alternative sources of authority to Constantine resided principally in their being opposed to him. There is no real case, ut opinor, for a dispassionate evaluation of the qualities of the emperor against his rivals taking place in Eusebius' writings.

But the positives outweigh these blemishes: by applying the study of demons in an author-specific content; by making it a little more difficult to treat Eusebius’ narrative as wholly triumphalist; and above all by encouraging scholars to treat such writings as having a complex pattern of causative elements that need to be investigated on their own terms, rather than merely quarried as mere historical source material.