Here's a key passage:
The grand tradition of philosophical religion thus aims at a symphônia of religion and philosophy. This term has a purely technical meaning, of course, but its cognate use in music captures the basic thought that we can harmonize the two voices. The guiding thought of Fraenkel’s study is that what may strike us as an unforgivably elitist distinction, between philosophers and non-philosophers, actually went along with a universalistic acknowledgment that diverse religious traditions share a common core. For it is precisely the social distinction between philosophers and non-philosophers that permitted philosophers to claim that, despite variations in literal content, religion bears an invariant allegorical truth—the insight that God and Reason are one. Plato, for example, believed that the laws of Crete and the laws of Sparta were essentially the same: variations in appearances could be explained by the philosopher as due to the influence of historical and cultural context. It was therefore possible for Plato, in Fraenkel’s assessment, to endorse both contextual pluralism (about variations in religious representations and practices) and universalism (about the inner meaning of religion itself).
Gordon's review is essentially a detailed précis of Carlos Fraenkel's new history of philosophical religions, and Gordon seems to share the view quite common among intellectuals that philosophical religion is a big improvement over ordinary religion because, so the argument goes, by placing religion within the sphere of civilized intellectual disputation you takes the violent edge off of the thing.
Following Fraenkel, Gordon speculates on why philosophical religion has declined — he simply assumes that it has, though perhaps Fraenkel provides evidence to support this claim — and what is noteworthy to me about those speculations is that they, like philosophical religion itself, operate strictly within the intellectual realm. It seems not to occur to Gordon that philosophical religion's fortunes might alter for reasons that are not strictly intellectual themselves.
Perhaps philosophical religion has declined (if it has) and has never been very popular (which is certainly true) because religion is not simply a matter of holding to a set of metaphysical propositions. Now, metaphysical propositions are intrinsic to most of what we call religions, but history would suggest that those propositions cannot be sustained without a strong framework of ritual and practice. This is something that all anthropologists and sociologists of religion know, and it seems like something that anyone writing about the fate of religion in a given society ought also to be aware of. Philosophical religion has never existed and never will exist in a vacuum: it always finds its place within a much larger set of beliefs, acts, and habits. A purely philosophical religion has never been sustainable because a purely philosophical religion isn't a religion at all.