As soon as one attempts to come to terms with the religion of ancient Egypt, one is immediately confronted by what I think of as the "transitivity" of Egyptian divinities; Hornung refers to it as their "vagueness." The gods were continually combined, recombined, made equivalent to one-another, interpreted as aspects of one another, schematized, and re-schematized in ways that make it difficult to think of them as having a single office or a stable identity. This is not merely a problem of historical development; frequently, a single god will be described in contradictory ways within a single text.
Unraveling this problem is the focus of Hornung's book, which is one of the foundational texts of modern Egyptology. Specifically, he wishes to address the apparent contradiction between references to God in the singular and gods in the plural, which are frequently found side-by-side in the same text. Hornung quotes Le Page Renouf, who described the problem well:
"Throughout the whole range of [ancient] Egyptian literature, no facts appear to be more certainly proved than these: (1) that the doctrine of one God and that of many gods were taught by the same men; (2) that no inconsistency between the two doctrines was thought of."
Making sense of this state of affairs is the purpose of the book, and to do so, Hornung goes back to the basics, and makes an exhaustive study of what we know or can deduce about how Egyptians viewed gods, what they meant by the term. The majority of the book comprises this detailed review which covers every conceivable aspect of divinity, including the hieroglyphs used to indicate them, the pantheons, descriptions of their appearance, their schematization into groups, and so on.
Hornung's knowledge is vast, and this detailed survey is the strongest part of the book. It contains a number of highly illuminating observations, such as his argument that we should not think of Egyptian depictions of deities as "pictures of what they looked like," persuasively arguing that they should instead be regarded as ideograms that reference the deities and indicate their general attributes in a pictorial language. No Egyptian, he claims, would have believed that Re actually had the head of a ram.
Now, here he evidences a concerning tendency to over-generalization. Truly, no Egyptian, in thousands of years of history? Knowing what I know of religious psychology, I will state with 100% certainty that plenty of Egyptians believed Re had the head of a ram, and didn't particularly trouble themselves about it. It is quite common to adopt a naive and literal reading of religious texts.
As interesting as this book is, I would say the majority of his review of characterizations of the gods is a digression from his main argument. His actual solution is not very complex, and could have easily been argued and demonstrated in an essay of about 15 pages. In this book, to a large degree, the journey is the goal.
His solution is based on his fascinating reading of Egyptian ontology, which holds that for the Egyptian, the very concept of existence entails plurality. The only thing that is really unitary is the undifferentiated, inchoate primordial reality from which all intelligible things emerged in the original creation, which is generally termed nun, or the primordial dark waters. Once things become determinate, they immediately take on an irreducibly plural character, which is why, for example, Egypt itself was called the "Double Kingdom," even millennia after Lower and Upper Egypt were united.
So to be is to be manifold, and this is the case for divinity as well. The Egyptian, then, works out a kind of theological and mythological algebra that allows them to articulate divinity as manifest in various forms. This position, which he describes as henotheism, borrowing from Friedrich Schlegel and F. Max Müller, essentially means that any of the numerous gods are themselves the totality of divinity.
Hornung seems to agonize over this characterization, and seeks to find a way to argue that such a position is in fact "logical," even if on the surface it appears to contradict the law of the excluded middle. Perhaps there are non-standard forms of logic, he writes, with which such a worldview can be made to accord - something suggested by quantum mechanics or the complementarity principle.
Here I think his near-total lack of a comparativist perspective really harms his analysis, for this conception of divinity will hardly appear to most readers of this book as novel and incomprehensibly strange. Christian theology is not technically henotheistic, but the difference between the Nicene interpretation of the Trinity and henotheism will only be comprehensible to specialists. So the western church presents us with a godhead that is three persons with one nature, and most lay Christians don't seem to lose much sleep over that, even if they don't have a clue what it means. Most people, unlike Hornung, simply take it in stride that we're talking about a different regime of reality, and that saying "there are three persons in the Trinity" is not like saying "there are three loaves of bread in this basket."
Hornung is curiously averse to allowing for such a stance, allowing that the domain of religious imagery is characterized by a different kind of logic. I suspect this has something to do with his insistence that for the Egyptians, the gods are entirely within the created order, and in no sense "transcendent." I am not certain how he reconciles this with his statement "What a god is cannot be defined. Whatever statement we make about him [sic], it does not exclude a mass of other statements." (258)
It seems pretty clear to me that Hornung's affinity for schematization and to rigorous detail often gets in the way of his larger arguments, and often takes a toll on his readability. Nevertheless, I think the core argument of this book is persuasive, and even if much of it was tangential to that core argument, it was still very much worthwhile.