As it seems to be defined in the passage, less-than-devout means not as religious. In the 2nd paragraph, the author is making a contrast between the Church music foundations and saying it is still admired, even though society may not be as devout (i.e. religious) as it once was.

I don't see how A relates at all to that idea. Looking at the pyramids has nothing to do with devotion to Church. C, however, is directly saying that few people attend religious services, which would be less-than-devout. Since C is directly related to the quote in the question, it's a good answer choice.