One commenter on the previous post wanted me to say a bit more about why I don't think religion can deliver objective prescriptivity any more than secular ethics can. That's a large question, so I'm not going to say a lot today. It would require a long post to nail it down. Also, there are many religious theories of morality. However, to give an idea, consider the sort of crude divine command theory that says something like, "Actions (and inactions) are morally obligatory if they are commanded by God."
This notoriously leads to the question of whether they are obligatory because they are commanded by God or whether God commands them because they are morally obligatory (on some other ground).
The former is a problem because somebody can ask why she should simply do whatever God tells her to do. After all, at this stage of the argument she can imagine God telling her to torture puppies, kill people, or whatever other horrible action you care to imagine. Surely our ordinary concept of morality cannot make these actions morally right or obligatory simply because they are commanded by a very powerful being. If a god commanded such actions, we'd be more inclined to think this was an evil god than that the actions were now morally obligatory. I concede, of course, that a powerful being might be able to offer us rewards for obeying its will or threaten us with punishments for disobeying, and that can give us a reason to obey. But this reason appeals to our desires (for rewards and for the avoidance of punishment). So acting in these ways is not objectively required - i.e. it's not required in a way that transcends our desires.
What if we say that God would not command such things because God only commands things that are already morally obligatory on some other ground? God isn't, on this approach, originating what is morally right and wrong, etc., but enforcing it. Again, the fact that God may be able to reward or punish us gives us an extra reason for acting morally, but (as above) it's not one that transcends our desires.
The question is then whether whatever else it is that makes acting in these ways morally obligatory can give us desire-transcendent reasons. But that "whatever else" is going to be something that's also available to an atheistic moral theorist. So the divine command theorist has no more resources available than atheistic theorists to find reasons that transcend our desires.
There's a lot more to say, not least because I've only discussed a crude theory of religious morality. So in the space of a blog post I'll just have to assert that more sophisticated theories will run into similar problems. Sooner or later, the theorist will have to be asked what reasons are available, and will be no better equipped than someone coming at the issue from a secular viewpoint to find reasons that transcend our desires.
I was also asked to recommend some books that define and discuss such concepts as objective prescriptivity, motivational internalism, etc. - i.e., the sort of language I was tossing around in the previous post. The ideal book here would be a well-written undergraduate-level metaethics textbook. Unfortunately, I don't know what I'd recommend. I suspect that a lot of courses in this area would depend on a collection of readings more than a text. Can anyone help out with this question?
A commonly used undergraduate text that goes beyond metaethics into wider areas of foundational moral theory is James Rachels, The Elements of Moral Philosophy. This is very clear and accessible and covers a lot of ground, but it's maybe not advanced/specialised enough for the particular purpose. Also, I have misgivings about relying on it, because Rachels pushed his own views pretty hard and tended at times to deal with simplified versions of views that he didn't like (doubtless in good faith - see above where I myself deal with a rather simple version of religious morality). All in all, the Rachels book is a good starting point, and I do recommend it, but it should be taken with at least a small grain of salt. Its criticisms of subjectivist moral linguistics and of social contract theory, for example, appear to me not to meet these approaches at their strongest and/or to make controversial assumptions.
More specialised metaethics texts that I've encountered tend to be aimed at a level that's too advanced. I'm still trying to think of a book that falls nicely in the middle.
If you've got a solid background already, a good approach would be to plunge straight into The Moral Problem by Michael Smith and then The Myth of Morality by Richard Joyce. These disagree with each other, since one author is a moral realist of a kind and the other is an error theorist. Although my views are closer to Joyce's, all three of us actually agree on a lot of things, especially about how the issues should be framed. However, be warned: both books are densely written. They require concentration and a fair bit of prior familiarity with moral philosophy.