What was later called expressivism about ethics, and what we can call Wittgensteinian approaches to religion, had their origin in the same place: empiricist theories of meaning, which ruled out from “descriptive” discourse anything that was not empirically verifiable. In both cases, not everybody was willing to consign the discourse to the oblivion of pure nonsense; rather, the speech acts of asserting within the discourse were reinterpreted, and attention focused on the function of ethical or religious language, rather than its truth conditions. In ethics, this general viewpoint evolved into the work of Blackburn, Gibbard, Horgan & Timmons, et al, while in religion it had a good mid-century run as “Wittgensteinian philosophy of religion” and became ensconced in liberal Protestant theology.
I think it is fair to say (I’m willing to be corrected on this point) that most philosophers, and not least philosophers of religion, no longer regard Wittgensteinian approaches as the way to go in philosophy of religion. Rather, there are a lot of philosophers who think there is no god, and a fair number who think there is, and both are pretty clear about the truth-conditions of their claims. Reinterpreting obviously theistic discourse as not actually referring to a deity—as was characteristic of the Wittgensteinian approach—would likely be taken, among philosophers these days, as a form of unreasonable obscurantism, or perhaps bad faith. Since both sides of the debate now think of (some) religious discourse as descriptive, we ought to ask what made non-descriptive approaches so popular for a while, and why they fell out of favor. In particular, I wonder why they fell out of favor in religion much more precipitously than they have in ethics, where non-descriptive approaches are still very much in vogue.
It can’t simply be that, for a while, no one had realized the technical hurdles that non-descriptive approaches present to the semanticist, and when they did realize this the Wittgensteinians dropped their project, because the hurdles are no harder in religion than they are in ethics and the expressivist project in ethics is still going strong. In fact, I don’t think anyone has pursued Frege-Geach style objections to Wittgensteinian philosophy of religion, though clearly they could. There is some other explanation at work.
Let me try a sociological explanation. (I should be careful to note that I am not one of those who think that explanations of beliefs in terms of non-rational factors are necessarily derogatory explanations.) There was a while, in the middle of the twentieth century, when respect for and attachment to religious institutions was high among educated people, while belief in their metaphysical underpinnings was low. This produced cognitive dissonance, which was resolved by turning to non-descriptive ways of understanding religious discourse. The alternative was thinking of too many of your friends and relatives and colleagues, not to mention other cultures, as whacked; also, perhaps, giving up a practice you were happy to continue in. But as these sociological obstacles to thinking of religious discourse as straightforwardly false have disappeared, so has the attraction to Wittgensteinian philosophy of religion.
Suppose something like this is the right story. Turn now to expressivism in ethics. The parallel explanation (which, again, I do not intend derogatorily) would be that expressivism is what you get when people remain attached to the practice of making moral/normative/evaluative claims, but have lost belief in any underlying metaphysics that would make those claims true. Unlike religious discourse, however, there is no chance whatsoever that people are ever going to disentangle themselves altogether from normative/evaluative discourse. So they continue to resolve their cognitive dissonance, aka philosophical problem, with expressive interpretations of ethical claims.
I am not sure what, if anything, these ruminations provide an argument for. Maybe it breaks down like this. The parallels between the cases of ethics and religion support moral descriptivism: we’ve come around to descriptivism about religious discourse, so we should about ethical discourse too. The differences in the two cases—in particular, the impossibility of abandoning ethics in the way one might abandon religion, even if/when one thinks that the metaphysical status of the two realms is on a rough par—support moral expressivism of some kind.