P. Z. Myers has recently been in a bit of a flap over his claims that religious belief is foolish and irrational. I've also been hanging around with Dan Dennett's goddaughter a bit, and Dennett of course pulls no punches when it comes to criticizing religion. I have myself thought about the content of religious claims before, as I find it to be a profound puzzle how anyone can take them seriously, and recent events have gotten me back on that subject.
Most philosophers know that the old positivists were in the habit of dismissing religious claims as meaningless, devoid of cognitive content like the various metaphysical claims they so famously opposed. In fact, that is slightly oversimplified, as they standardly distinguished between what they called mythological religious claims and what they called metaphysical religious claims. The terminology may not be ideal, but I don't have any really good replacements, so I'll stick with their terms.
Mythological doctrines essentially involved treating the tales of gods and miracles as historical claims, which of course we have every reason to think are false. This is the version of religious doctrine which Myers and Dennett so like to mock. The positivists, on the other hand, paid virtually no attention to this interpretation of religious claims. Partly this is because they saw religious philosophers as more prone to the metaphysical than the mythological, and partly this is because they saw many religious believers as equivocating between the mythological and the metaphysical, but I think there is more to it.
On the metaphysical interpretation, religious claims are, according to the positivists, devoid of cognitive content. There is no test to determine whether the god of the philosophers, that metaphysical abstraction, exists or not. It is very unclear what the claim that such a god exists even means; the religious philosophers themselves often emphasize the mysteriousness of the absolute. The positivists argued that it's so hard to interpret the metaphysical religious claims because in fact they have no content at all; they aren't really saying anything which could be true or false.
Now, it is not clear why it would ever be a problem that people sometimes make meaningless noises. We already knew that. So a little appreciated question is why the positivists considered metaphysics to be in need of being attacked, of being criticized, if really it said nothing at all. If it's all just babbling, wouldn't that make it harmless? The reason the positivists thought otherwise is that they didn't think metaphysics lacked any content whatever. According to the positivists, metaphysical claims encoded disguised values.
This is why viewing metaphysical claims as true or false was, according to the positivists, anything but harmless. The encoding sought to put the values above criticism. Since they were being presented as truths, the usual examination to which we subject values (contemplating what we really want and trying to coordinate our aims with one another and with those of other people) are ruled out; truths are not matters to be negotiated. But since they weren't really truths, they couldn't be tested as truth claims usually are either; no such tests were applicable. Thus, the metaphysical values could not be criticized at all, and it is this attempt to conjure up absolute values and shut down any possible challenge to them which the positivists found profoundly dangerous and harmful.
I think this explains why the positivists focused on the metaphysical aspects of religion. We read polls that more than 90% of Americans believe in God in some form, and far more specific and empirically questionable claims are also endorsed by very large numbers. But I think that's misleading. It's questionable to me that a majority of those people really believe what they claim to, at least in the sense that they believe, for example, that ripe bananas are yellow or George W. Bush is president. As Hume noted, no Christians act as if they believe they'll go to hell if they sin. Very few of them will go to faith healers rather than doctors if they're sick. Very few of them consult religious authorities on any factual questions about how the world works which are relevant to their lives.
I think even for the religious rank and file, the religious claims are metaphysical in the positivists' sense. The strange, crazy religious doctrines are value judgments of various kinds, disguised as truths. Thus, they are not treated as truths by most of the believers in everyday life. Still, they are presented as truths, and this disguise plays a vital role (insulating them from being seen as evaluative preferences, subject to negotiation). Thus, criticizing them as truths still challenges them, and the religious will respond fiercely to any such challenge. They will be all the more passionate about it since it is their values that they see as really under attack, but they have to defend the claims as truths in order to maintain the edifice of disguised values.
Of course, this diagnosis of what's going on does not immediately indicate what to do about it. Since religious values are often deeply problematic, and confusing truths with values probably does have some tendency to foster other confusions about truth, I agree with Myers and Dennett that it would be profoundly preferable if more people could be freed from viewing religious claims as true in any sense. But the fact that religious doctrines are not held for anything like the reasons most factual beliefs are held makes it hard to know how to question them; questioning their factual basis seems like it won't work very well unless the factual basis is actually relevant to the holding of the belief. Still, other alternatives are not obvious either.