The book covers in very broad, sweeping terms the development of monotheism as a religious idea tracing the concept from an Egyptian Pharaoh, to an assumed historic Moses, to Josiah and then to Christianity as it was between the first and fourth centuries. Parallel to this, Kirsch does a decent job of providing a glimpse into the polytheism which pervaded the ancient world, especially the variety of Roman polytheism around the third and fourth centuries CE. He also examines two influential historic figures, the emperors Constantine and Julian, as well as the culture, society and politics both developed in. The bulk of the last half of the book actually focuses on each Emperor and how they in essence, shaped the course of Western religious history. His basic theory of the development of monotheism traces the political aspirations of a given monarch and then parallels that with a desire to institute the worship of the "One True God"; in essence if there is a single all powerful deity, than there ought to be a single all powerful ruler. Another strain of thought running throughout the book, and perhaps its actual thesis, is that contrasting the inherent tolerance of polytheism, with the inherent intolerance of monotheism.
He also does a decent job of putting the so called Christian persecutions, in their historic and cultural context. While he does not downplay the significance or immorality of the mass murder of people who held different beliefs, he does explain the reasoning behind them. On top of that he is also critical (as are most modern historians) of many of the accounts of Christian martyrs, and examines the hyperbole in a number of martyrologies. Further, he also points out that the number of Christians killed by the Pagan Romans over the "10 persecutions" pales in comparison to the number of Christians killed by the Christian Romans in the subsequent centuries. Mind you, any decent text of the period ultimately does the same, historic facts and all that. Though I could understand why many more literalist Christians may find offense with the text, and probably argue that Kirsch is trying to downplay the persecutions. As I said, the facts speak for themselves.
Now for the criticisms. This is by no means a perfect work, and I hinted at its tendencies to gloss over a lot of the details and probably oversimplify any number of issues. My own knowledge of the period and culture is significantly less than my expertise in other areas, so I can not comment greatly on the portrayal of the Religio Romana, and I do question his assertion that traditional polytheistic religion was being replaced with so called "Pagan monotheism", this tends to be a problematic perspective a good number of religious historians suffers from. Though I do think his assertion that mystery cults were gaining in popularity and mass appeal has significantly more merit. My biggest criticism is his portrayal (albeit brief) of the Celts. He mentions them, almost in passing, as an example of the more "barbarous" forms of polytheism:
"...or the Celts of Britain, who enclosed their human offerings in wicker baskets fashioned in the image of a god and then lowered the basket into a bonfire. Such pagan luminaries as Pliny and Cicero condemned these practices, and the Roman generals who conquered the barbarians and occupied their tribal lands expended much effort in suppressing the practice of human sacrifice"
It is unfortunate in that a well researched and adequately footnoted book, such an ignorant and uncritical statement is used to shore up the image of Roman polytheism. I understand that his focus was on the Roman sources, but considering how critically he tends to read the sources, especially when they mention other religions or cultures in a negative light, I was caught off guard by this paragraph. It really was, for me, a blight on an otherwise well reasoned text.
He also tends, unfortunately, to rely on a number of antiquated and outdated texts when examining interpretations of many of the myths and commonalities of the myriad polytheistic religions. He seems to enjoy works by Campbell and Graves a little too much, and a little too uncritically.
Overall I would recommend this text to anyone who is interested in the subject matter. I will also admit that there are better books on the relationship between polytheism and Christianity within the context of the Roman Empire. I will admit, however, that what I enjoyed most about this text is Kirsch's willingness, and perhaps even earnestness, in showing polytheism in such a positive light. Books on polytheism in general are few and far between, and books which show the merits of a polytheistic world view are even fewer. I mentioned before, but even those well written and argued historic texts which examine in greater detail the religion of the Romans (or other polytheists) treat it as something which was inevitably doomed to fail, in the face of the "One True God". Kirsch, at least, illustrates how history could have so easily gone another direction.