I guess I'm not so easily put out when it comes to mentioning religion since I'm not American. "We" usually see between the fingers when talk about religion comes up in tv shows from the US, we know it's a bigger deal where you're at than it is here. So when God get mentioned we just shrug and kind of ignore it. ;)
I do agree it's a bit out of character for him, but then - meeting Castiel has to have made a good sized impact on Dean. Just becasue he doesn't believe and secondly, with all his knowledge he had never heared about ACTUAL angels before.