With Stephen Prothero's book God Is Not One still causing waves in religious writing online, I am going to put forward some thoughts that are probably completely unoriginal.
I haven't had a chance to properly read Prothero's book, but based on summaries, blurbs, and reviews, I get the feeling that while he acknowledges commonalities such as the Golden Rule and the similar experiences of mystics and contemplatives, he also worries that specific differences between theologies, dogmas, rituals, etc are being over-looked or minimized. Those who have used Prothero's book echo this sentiment as well as the belief that treating all religions more or less the same is both disrespectful and dangerous. Dangerous because it obscures differences that can fuel conflict.
Perhaps what I'm going to write here is something covered already in God Is Not One, but is that such a bad thing? Again, I am not writing in response to Prothero but to the conversations online his book has inspired.
First up, yes, there are difference in the narratives of different religions, and different versions, interpretations, and emphases on the dominant narrative within an individual narrative. Yes, such narratives include variation and ambiguity that allows for rituals and texts to be used for different purposes or to come to different conclusions about existential mysteries and daily life. Yes, glossing over these differences is problematic when they are important to particular practitioners or groups of practitioners. So no argument from me here on that score.
Second, there is more to religious commonalities and distinctions than this. Which is where I may disagree with both those who insist that God is one and those who insist God is not.
More Religious Distinctions
I would suggest that we keep in mind that religion tends to refer to a mythical narrative (myth here referring to key stories that tell us about existential truths of the human condition) and its associated practices and institutions that arise in societies with intensive agriculture and at least the early vestiges of state level societies (or civilizations as they are classically known in archeology). That is, after subsistence strategies and settlement patterns allowed for individuals to become full time specialists in service to society through its rulers, therefore allowing for a formal priesthood rather than the shamanic diviners and healers common to small-scale societies.
What, then, do we call the mythical narratives and its practices and social institutions in small-scale societies or those transitioning to the larger, more complex form of social organization and stratification of civilizations? It is often just lumped in as "religion" as well, but if we are going to take a more nuanced approach to the topic we should also recognize such differences as well in how "religion" is structured, perceived, and practiced according to the size and complexity of a particular community.
There are differences between a religion of a larger society or state and that of a cluster of small clans. The unseen forces shaping, guiding, or controlling the world take on different forms and significance. The social function and organization of the tradition can be vary. And that's just to start.
The point isn't to add another layer of distinction to deal with, but to highlight that people in different societies, cultural communities, and so on may understand what people in postmodern industrial societies label as religion. For example, the idea of segregating such aspects of worldview and social life into its own sphere called "religion" separate from other aspects of human existence isn't the norm everywhere. It's just part of the fabric of reality.
Nor can we ignore the fact that people in larger societies often impose their own assumptions about religion as they have experienced as generalities true of all religion: that it involves personified divine beings, that it involves an afterlife, and so on. Attempts to over-generalize are highly problematic. My own broad framework, about a mythical narrative and its associated practices and social institutions, isn't a bad start but it defies expectations about all the other bits people take for granted as being necessary for religion. Even adding in unseen or cryptic forces that shape, guide, or control the world still leaves thing very vague. But not so vague as to be without usefulness, as it lets us appreciate that anything directly and intimately tied to such elements of cultural tradition are assumed to be make up the "religion" of a particular community.
When people in the post-modern world talk about religious unity or universality, they refer to common ethical standards or imply that the forces at work behind the scenes are in fact the same. By using one or both of these assertions, there is an assumption is that these are what really matter the most about religion. This highlights a crucial point -- not only does ecological, social, political, economic, and cultural environment interact to shape what we are loosely referring to here as religion, but also how individuals see the point and value of religion. This gives us another way to think about religious similarity and difference.
Categories of Religious Perception and Participation
While this perspective could be filled with a number of categories, I will use four here. One is the spiritual category. By spirituality I am referring to the impulse for seeking meaning and therefore purpose, typically through the pursuit of self-transcendence by incorporating oneself into some larger structure or system offering such meaning. This does not necessarily involve following a religious tradition or believing in what are called spirits. Rather, spirits reflect a set of beliefs deriving from this impulse. Religious people in the spiritual category often seek out hidden mysteries, esoteric knowledge, and are driven by curiosity, a sense that there is "something more" to existence, or a longing for something greater to life. Rituals and other practices allow one to be a seeker.
Then there is the kinship category, which we could also call the social identification and affiliation category. Here, the quest for a sense of meaning or purpose is sought out by relationships and placing oneself within a lineage that identifies with some mythic founder. Mythic here doesn't mean "not based on a real historic person", but that the person is a key figure in the religious narrative and is often described in exaggerated and symbolic imagery. This category emphasizes belonging (and not belonging) to the religious community and this is useful in signalling not only your beliefs but your allegiance and family history. Rituals and other practices distinguish you as a member of a particular tradition.
Next up is the political category. All of these categories overlap, and there is certainly overlap with the kinship category, but here the focus is on social influence. That is, on power. On how relationships reflect relative power to others and the source of authority for that power (sanction of divine beings, possession of magical powers, etc) and how it plays out in the community. This can be coercive or persuasive power, enforced by societies governing institutions, by acceptance of the religious tradition's authority in determining leaders, or by individual charisma. Rituals and other practices demonstrate and validate power relationships within the community.
Lastly is the magical category. The main attraction here is the ability to influence or control the hidden forces of the universe. This might be for material desires or social ambitions. It might also be used to placate or please the cryptic powers. The existence of those forces and their ability to be influenced by human activity and in turn alter something about reality is crucial. Rituals and other practices allow one to influence hidden forces to achieve a desired outcome.
Now, again, these categories are not mutually exclusive and any particular religious belief and behavior will involve more than one of them, but the goal here isn't to see how a belief in being saved from hell, for example, puts together elements from different categories of religious experience and perception. Instead, it is to suggest that the emphasis for any particular religion or a specific school or sect within that religion can focus more on some of these categories and less on others. Also, individuals within a religion may focus more on one or two of these categories over the others.
So, if you or your sect focuses on the kinship and political categories, the importance of the differences in tradition, distinctive beliefs and practices, and so on that set you and your community apart will be emphasized. The idea of all religions being more or less the same becomes foolish and insulting.
If the focus is on the spiritual and religious categories, especially the former, the relaxed attitudes about distinguishing groups and focusing on power relationships and authority (both of the community and within it) allows for a more universalizing attitude. It's more plausible that there the same forces are at work behind all religions which are then filtered through individual cultural communities.
In other words, there can be Buddhists, Christians, Muslims, and Wiccans who all stress the importance of the differences between their religions and focus on those details, and there can be members of those religions who see each other as kindred spirits on different paths to the same goal. Neither attitude can be said to be more or less genuine or authentic in terms of being religious.
While it may be true that differences are sometimes ignored or glossed over, the important thing is to appreciate to whom this matters and why.
God (and religions) are and are not one. So there.