My best guess is that we can just chalk this up to the general dynamic of recession-induced suspicion and incumbent unpopularity.Possibly. I have another, complementary, theory: no one cares.
When there's a lot going on in your life and you're stressed about losing your job or finding another one and the bills are piling up, you don't care what religion the President is. Spending time caring/screaming about irrelevant facts is a normal good for some: it rises as income rises. In the early days of the recession, when some thought it might be a small blip, people cared about this. They were willing to listen beyond the occasional yelp that he's a secret Muslim, sometimes onto more reasonable people that correctly point out he's Christian. But now they hear those screams, brought up again surely because of the midterms and because commentators know people aren't going to look into it as hard, and accept it. Because double-checking is a pain, thinking hurts, and they have better things to do.