I'm a third of the way through this book, which attempts to establish the superiority of an atheist worldview to a Christian worldview when it comes to providing an objective morality and a meaningful life. So far, the author has been trying to establish a non-religious basis for moral objectivism. He's going to go on and try to show that there's no good religious basis for it.
If I were reviewing the book at length, I'd have many of the same objections to it as I have to The Moral Landscape. Once more, that doesn't mean it's a bad book; it simply means that (in many cases) the most interesting things that a moral philosopher can say about a book of moral philosophy relate to possible objections.
So at this stage, let me just say once again that the ordinary conception of morality is like the proverbial carpet with a bump in it. Morality can't be all the things that it normally looks like it is. If you try to remove the bump, you'll shift it somewhere else. In this case, Martin offers a system that he claims gives us an "objective" morality. It gives us moral properties that are out there in the world. Fine, it's a complicated sort of moral naturalism (based on Objective Observer theory). But he accomplishes this at the expense of throwing away motivational internalism and, more importantly, objective prescriptivity. He explicitly disclaims these.
The result is that the "objective moral properties" don't actually bind anyone to do anything. Somebody who is not mistaken about any of the non-moral facts, or indeed about any of the moral facts that Martin offers, and who makes no mistakes of reasoning, may, depending on her desire set, go ahead and do whatever she wants, perhaps muttering, "What is that to me?"
So, yes, you can have your objective moral properties if you're prepared to accept that there's no objective prescriptivity (and you can abandon motivational internalism for good measure). But in doing so you merely move the bump in the carpet. It's objective prescriptivity that really matters - without this, morality is not rationally binding on anybody. When error theorists claim that first-order moral judgments (or at least thin first-order moral judgments) are tainted with error, it's because they see these claims as being or including claims about objective prescriptivity.
So, again, yes, you can a system with objective moral properties but no objective prescriptivity. You might think that objective prescriptivity is so metaphysically peculiar that it can't be what is meant in ordinary moral discourse, so you're prepared to give it up or to say that it was never part of our moral discourse all along. I beg to differ - it looks to me as if it's right there at the centre of everyday moral thinking, and that everyday moral thinking is, to that extent, philosophically problematic. A system such as Martin's offers only an ersatz moral objectivity: it may have objectively existing natural properties, even though they're rather contrived ones, but morality does not end up being objectively binding on anybody.
But of course, religion can't provide objective prescriptivity either. It can also provide only an ersatz version of moral objectivity. One of the (possibly) disconcerting truths that we just have to live with is that objective prescriptivity is not a feature of the universe - and in fact when you push hard enough on the idea it doesn't even seem to be coherent.
And yes, I'm quite happy to say this in the public square on occasions when it's possible to explain it properly. I do agree with Jean Kazez that it's not the sort of thing that you can necessarily describe in a brief newspaper article - unlike the things you can say about science/religion (in)compatibility. Even this blog post (which will be read by a few hundred people at the most) is going to be rather cryptic for those who are not familiar with the terms I've been throwing around. But it's still worth making the point when the opportunity arises, for those who are prepared to think about it.