The aim of this book is twofold: first, to answer the question why John of Damascus (c. 650–750) called Islam a “heresy” and included it in his century of heresies (chapters 1 and 2); and second, to defend John against the charge of having misunderstood or distorted Islamic faith and doctrines (chapters 3 and 4; chapter 5, which gives an outlook on Theodore Abū Qurra, cannot be further discussed here). The former goal has been fully achieved, the latter has not – or at best only partly. Being a historian and an expert in Christian heresiology, Schadler gives a thorough analysis of what is called “heresy” in antiquity. The word αἵρεσις originally meant “choice (of a certain doctrine)” and can be applied to scientific (especially medical) or philosophical schools as well as to religious groups. Given a semantic range wider than is usually assumed today, a “heresy” is not necessarily a schismatic movement, and a “heretic” is not necessarily a former insider apostatized from an “orthodox” belief. In the framework of John's encyclopaedic Fount of Knowledge (of which the book of heresies forms the centre part), the chapter on Islam is just the last item in a complete collection of ideologies. This means that Islam is not simply a Christian “heresy” (a view based on the famous Sergius-Baḥīrā legend), nor a philosophical school, but a false religious doctrine (θρησκεία in Greek) which is, in a way, opposite to the “common notions” (κοιναὶ ἔννοιαι) of human beings, i.e. self-evident truths. This implies, however, that John regarded Islam as a phenomenon showing a certain degree of unity, at least insofar as single rites, doctrines or lore could be identified as “Islamic” in the first half of the eighth century.

The second part of the book is much more problematic. The crucial question is whether John, who certainly knew Arabic, knew the Quran in its canonical form. The obvious answer seems to be “no”, but it is in fact more complicated. Even if John did not know the Quran in its canonical form, this does not mean, as Schadler suggests, that a canonical Quran did not exist at John's time. In the light of recent scholarship (especially on the Sanaʿa palimpsest and the Birmingham manuscript), there can be little doubt that the text was fixed c. 700 at the latest (probably as early as the mid-seventh century). John, however, obviously mixed up Quranic and extra-Quranic material, quoting, for example, a non-existent surah named “The She-Camel”. The term γραφή used here and elsewhere to denote “surah” is also applied to the whole “book” (βίβλος) of “Mamed the pseudo-prophet”. What else could John have meant by these terms than the Quran? Schadler concludes that John's deviations from the Quran as we know it today point to a different version, or versions, of the Quran in the eighth century.

The evidence found in John, however, by no means supports a “revisionist” view of early Quran text history. So how do we explain this evidence? Schadler is quite right in rejecting the notion of a deliberate distortion of Islamic doctrines or customs by John, who did not want to distribute “fake news” about Islam, but to give “authentic” information on it. However, we must not forget that John intended to refute Islam, which he called “forerunner of the Antichrist”. To this purpose, John indiscriminately used different sources – written texts (be they part of the Quran or not), oral traditions and ritual performances. Having his Christian readers in mind, John did not care whether a ridiculous story (like that about the she-camel) was reported in the Quran or elsewhere. In John's view, the Quran was not a holy book or a canonical work, but a compilation of the alleged prophet himself, and therefore its text is not sacrosanct. One detail suffices to illustrate this. Right at the beginning of his description of Islam a central dogma is quoted, namely Q. 112: 3 lam yalid wa-lam yūlad (“God has not begotten nor was he begotten”). The Greek runs the other way round: μήτε γεννηθέντα μήτε γεγεννηκότα (“that he was neither begotten nor had he begotten”). Schadler argues that the reverse order of the verbs points to a different (or not fixed) Quran text. This is quite unlikely, since this anti-Christian dogma is written most prominently at the Dome of the Rock (692) and is attested to have been engraved in a tomb as early as 650. The explanation is, rather, that John – deliberately or not – restores the logical and chronological order one would expect in a theological dogma on God's transcendence (first that he himself was not begotten, second that did not beget). Of course, John is not scrupulous to alter an – in his view – unholy text.

Generally, the book represents an outdated state of research. In the bibliography and in the footnotes, items from after 2010 are absent, and even for the years 2009–10 some important titles are missing: A.Neuwirth, Der Koran als Text der Spätantike (Berlin, 2010) is not cited; B. Roggema, The Legend of Sergius-Baḥīrā, is referred to as an unpublished PhD thesis from 2007, although it was published in 2009 in the very same Brill series. On further titles, see my article on John of Damascus in EI³ (Schadler cites only EI²).

In addition, the book should have been typeset more carefully, especially with regard to Greek characters (there are many false accents and aspirates: αἴρεσις, for example, occurs several times). Arabic has been transliterated, but mostly without diacritical signs.

In sum, Schadler has made an important contribution to the understanding of John's place in the theory and history of heresiology. He also offers interesting insights into how Christianity saw the early stages of Islam, at least in Syria/Palestine. However, it is a serious shortcoming of the book that it has not been updated since 2009–10 (Schadler was awarded his PhD at the University of Oxford in 2011), especially regarding the early history of the Quran text, a field of research evolving rapidly. In the light of recent scholarship, Schadler's interpretation of John's De haeresibus ch. 100 needs some rethinking.