That's much less an issue now than 50 years ago though, which is what's important when talking about a relative decline. It was much less socially acceptable to identify as agnostic in 1960 than it is today.
But I think what the original poster intended was the decline in number of Americans attending a traditional church or believing in Christianity.
That shift is fairly pronounced, especially over the last two decades.
I was definitely speaking from my own social milieu (young to middle age tech workers).
In fact, the line from Christianity to work ethic is still so indistinct, many things would have to happen in order for us to make the jump to working less.
On the other hand, if the current younger generation is as unemployed as I understand it, then perhaps working fewer hours will simply be their default choice.
Where did you get that from? A quick google search says about 3/4 of Americans identify as Christian, and about 62% are members of a church congregation.