The point is simply that a morality predicated on Enlightenment rationalism retains its Christian foundations, at the expense of coherence. Therefore the moral codes we retain after the death of God are grounded in nothing, a point the Neo-Darwinians underscore every time they trumpet that article of faith, the “morality gene.” It is not enough to argue that we can simply ground our morals in ourselves, in our conceptions of the good (for one thing, it is self-evident that we don’t agree about what these conceptions should consist in).This is bullshit on several counts.
That religious people of the past were often quite as murderous and duplicitous as we is beside the point, properly understood. We are talking about the loss of a coherent worldview, about grounds, not about practices. Anyone interested in the history of the shaping power of mental conceptions should understand why such a loss is a problem.
First, it is just calumny to claim that "Neo-Darwinians . . . trumpet that article of faith, the 'morality gene.'" This statement is a pure straw man. There's no "morality gene," and the genetic basis for morality is hardly an article of faith. Morality has a genetic basis in the same sense that everything has some sort of genetic basis: morality happens in our brains, which obviously have a genetic basis. We have, for example, mirror neurons, which seem like a possible cognitive basis for empathy; these neurons have a genetic basis (which is almost certainly not a single gene). Simply dismissing opposing ideas as ridiculous is egregious intellectual dishonesty.
Second, the idea that "Enlightenment rationalism retains its Christian foundations" is at best controversial. It is, of course, true that Enlightenment rationalism (as well as modern welfare ethics) has an historical/genetic* relation to Christianity, just as Christianity has a historical/genetic relation to Roman paganism, which has a historical/genetic relationship to Greek paganism (and rationalism), and so on, probably back before the invention of language; chimpanzees seem to have a kind of morality without even a proper (Turing-complete) language.
*Not in the biological but the philosophical sense.
While a lot of the early Enlightenment thinkers retained belief in God, the whole point of the enterprise was to decouple morality, law, politics, economics, etc. from Church authority. But without Church authority, God lacks an authoritative voice. Once you undermine Church authority, you undermine any divine foundation for anything. For one thing, it is self-evident that we don't agree about what our conceptions of God should consist in. Indeed, the foundation of religious morality is not God, but the Church.
It is moronic to claim, "It is not enough to argue that we can simply ground our morals in ourselves, in our conceptions of the good (for one thing, it is self-evident that we don’t agree about what these conceptions should consist in)." Yes, we disagree, but has Robbins not noticed that human beings are clever and creative about developing methods of coming to agreement: e.g. argument, negotiation, persuasion, diplomacy, and sometimes force?
I suspect Robbins holds the all-to-common fallacy: morality is that which (among other things) is grounded in the transcendent; if something is not grounded in the transcendent, whatever it might be, it is therefore not morality. But of course there is no need to add the transcendent as an analytical component of morality. But of course we need not ground morality in the transcendent: we can in fact ground morality in human preference, utilitarianism, welfare, empathy and sympathy, pragmatism, natural intuition, categorical imperatives, or in any number of reasonable, natural bases.
Third, there is considerable controversy about what constitutes a ground of anything, whether we really need a ground, and whether a ground is even possible. Anti-foundationalism, for example, claims that grounds are impossible. The ground of logical deduction, for example, consists of premises, but what is the ground of the premises? It can't be deduction, but if it's something else, why not go directly to that, instead of using premises. Taking a different tack, dialectical materialism renounces the concept of ground: everything changes everything else; there is no philosophical starting place.
And finally, just because an intellectually dishonest doofus thinks that we have just abandoned grounding, or even that grounding is intellectually important, doesn't make it so. And grounding not really that important. "That's just the way things are" is a perfectly good, and perfectly trivial, ground.
ETA: Robbins argument fails on a more basic level. I don't think he's correct, but let's suppose arguendo he is correct: lacking "theistic belief" (whatever that is), our present ideas about morality become in some sense "incoherent." the incoherence of present ideas about morality would not by itself justify theistic belief: it could be the case that our ideas about morality are just hopelessly confused. If so, it would be rational to provisionally adopt historical morality out of pure expediency while we worked on a more coherent account. Even if theistic belief really could make our present ideas about morality coherent (which it cannot), theistic belief would still need an independent justification.
Robbins goes on to write, "New Atheists . . . wrote books that purport to challenge theistic belief as such. They therefore have a responsibility to address the best cases for God, not the dullest." First, we're not challenging "theistic belief as such," because that term is too broad and ambiguous to have any useful meaning. We're challenging certain kinds of theistic belief. Second, we have a responsibility to address the cases for God that are actually used. ETA:Furthermore, we have a responsibility to address the kinds of theistic belief that are most problematic; Robbins' beliefs, besides motivating an insufferable smugness, are not as socially or politically problematic as "fundamentalism"; besides, every competent economist and businessperson will tell you to go after the low-hanging fruit first.
Finally, the arguments that are actually used, mostly the Aquinian arguments, really are the best; the rest are inferior, and many just vacuous bafflegab. (I'm amused that Robbins considers Aquinas to be a dullard.) Robbins offers what he presumably considers a good, non-dull argument for God (without God, we have no grounding for morality), which I just challenged (and disposed of) a few paragraphs above. I am not the first.