We can see religion in two apparently benign lights: As cultural expression and as literary ethical metaphor. However, on closer examination, when interpreted specifically as religion these apparently benign expressions take on a more sinister cast.
Cultural expression are simply those arbitrary practices and rituals that are undertaken to have shared experiences specifically connected to one's geographic and genealogical relatives, past, present and future. One can attach no other specific meaning, for example, to decorating a Christmas tree, or arbitrarily choosing December 25th to exchange presents, than simply that other people are doing the same thing, have done the same thing, and will do the same thing in the future. In this sense, Christmas can be seen as a pure shared experience, undertaken at a specific time and in a specific manner for no other reason than to make it shared.
We can many supposedly religious practices as simply shared cultural experiences in this sense. Christmas, Eid, Hanukkah; church on Sunday, shul on Saturday or mosque on Friday; Thanksgiving, Canada Day or the Feast of St. Swivins; the business suit, dashiki, sari, or the keffiyeh. More meaningful, substantial interpretations could be attached to such events and practices, but there is a nontrivial sense in which we can view these practices as simply doing the same thing as everyone else for the sake of doing the same thing as everyone else.
Almost everyone participates to some degree in these shared-for-the-sake-of-sharing practices, and such practices play an important part in defining and reinforcing group identity.
In a similar sense, people in cultures have particular established ethical norms, and many of these norms differ from culture to culture. Furthermore, many of these norms are arbitrary choices, where consistency is more important than choosing correctly. We can see a trivial example of an ethical norm where consistency is more important than choice in the choice of which side of the road to drive on. It is much more important that everyone choose the same side than it is to choose the correct side of the road.
(I am not endorsing strong cultural relativism; we cannot conclude that consistency is more important than correctness just because some social norm is culturally established. I am, rather, endorsing weak cultural relativism: There are norms where we can independently establish that consistency is more important than correctness (or where consistency is at least benign), and different cultures socially construct the specific choice to establish consistency within that culture.)
An important method of establishing these social norms within a culture is by the use of literary metaphor. The culture employs fictional or mythological literary works to serve as ethical paradigms.
It is manifestly the case that some practices that people commonly label as "religious" serve these cultural practices. Many religious practices do in fact serve the purpose having something for everyone to do for the sake of everyone doing them: They are shared experiences to establish and reinforce group identity. Likewise, there is the sense that religious scripture is employed as ethical literary metaphor. These practices are common to almost all cultures, almost all people, and we have no reason see these practices per se as anything but benign and positively beneficial.
So, we should see the specifically religious examples of these practices as benign and positively beneficial, no?
An essential component of these cultural and metaphorical practices is that the content of the practice is unimportant. In a cultural sense, what we specifically all do is irrelevant: What is relevant is that we all do the same thing. Likewise to literary metaphor, the specific choice is unimportant; what is important is that we establish the consistency of the choice.
If the content of some practice or metaphor is relevant, then we can no longer look at the practice in the purely "cultural" sense; we must evaluate the content of the practice on its own terms. If some group of people tortures babies as a cultural shared experience — or uses literary metaphor to establish the obligation to torture babies — that they are torturing babies is more relevant than that this practice serves as a shared experience or cultural norm.
Looking at the history of all religions, especially the monotheistic Abrahamic religions, we see that the overwhelmingly predominant theme is the assertion of objective truth. It is not the case, for example, that Christianity establishes monogamous, lifetime heterosexual marriage as one manner among many to consistently manage the economics of childraising families. (I will assume, for the sake of argument, that lifetime monogamous marriage is or once was a reasonably efficient way to manage family economics.) We see, rather, that Christianity establishes lifetime monogamous heterosexual marriage as the correct way to conduct sexual and familial relationships, regardless of the pragmatic and instrumental consequences of that arrangement. Likewise we see the prohibition of interest in Islam (and early Christianity) not as a consistent way but as the correct way to manage second-level economics.
What earns some practice or metaphor the label of "religious" is precisely an assertion about the truth of content of that practice. But what makes a practice or metaphor a legitimately cultural practice is precisely the irrelevance of the content. Therefore, it is a contradiction to interpret any practice or metaphor as both cultural and religious.
It is unproblematic and trivially obvious that cultural relativism and diversity can be tolerated with regard to practices and metaphors where the specific content is (within some boundaries) irrelevant, but still maintain consistency of practices and metaphors within a specific culture.
It is vastly more problematic, however, to declare that the content is relevant, that one choice is correct, and any alternative is mistaken, and still tolerate cultural relativism and diversity. If God declares one practice correct and the alternatives mistaken, sometimes deserving of eternal damnation and torture, then to tolerate alternatives is, at the very least, to display a profound indifference to the welfare of others.
(Slightly less problematic, but still astonishing, is the idea that God himself establishes cultural relativism, that some specific content is mandatory for one group of people but irrelevant to others. What does that say about those outside the group? Does God (or the members of that group) consider the outsiders mere irrelevant, superfluous extras on the stage of history?)
Religion on the one hand raises irrelevant content to relevance; paradoxically, on the other hand it makes relevant content irrelevant.
If it is true that we should tolerate cultural differences on irrelevant content, then religion can, on the claim of cultural consistency, protect the consistency of otherwise relevant content by declaring it immune to humanistic or scientific scrutiny.
We see this sort of protection in the protection of Islamic misogyny and sexism, and Christian (and Islamic) creationist protection of scientifically false beliefs.
If some culture tried to justify the subjugation of women just on the basis of cultural consistency, they would be laughed out of court. Consistency, as noted above, is a justification only when the correctness of the choice is not at issue. But the subjugation and naked oppression of women shocks our conscience in just the same sense that chattel slavery or sati shocks our conscience; mere consistency is not a sufficient justification.
But by labeling a practice as religious removes the practice from ordinary, humanistic or naturalistic ethical discourse. The practice is mandated by God; mere human condemnation is irrelevant. Since mere human condemnation is irrelevant, the content is made irrelevant, and thus it becomes a legitimate "cultural" choice.
What permits cultural relativism, diversity and pluralism is the independent finding that the content of some cultural practice really is irrelevant. For many cultural practices, this independent determination is feasible or obvious. It really doesn't matter if we eat turkey on Thanksgiving, or fly kites on Eid, or eat unleavened bread at Passover. It really doesn't matter if we drive on the left side of the road or the right. It really doesn't matter if we prohibit or allow loaning money at interest.
But some things do matter, the content does matter, to any ordinary human being with a humanistic conscience, sensitive to the happiness and suffering of other human beings. The oppression of women does matter, and oppression and its symbols (e.g. the hijab or the burka) are objectionable, and cannot be protected as irrelevant.
And that's what religion always does, by analytical necessity. To call some practice both a personal choice and a religious choice seems either to make one's notions about God irrelevant and meaningless (God says do this, but it's acceptable to do that?) or to engage in discourse that is not only disingenuous, but positively mendacious and hypocritical.