The Handmaid's Tale by Margaret Atwood
The only one of my Five Faves written in my lifetime (1985 publication), this novel extrapolates American sociological currents first spotted in the 1980s, currents that have since become waves, to logical (if horrifying) conclusion.
The opening paragraph of a NYTimes (login required, copy here) interview with Atwood that I can't beat sets the stage:
The President and Congress have been assassinated by right-wing religious fanatics who have overthrown the Government and set up a monotheocratic [Christian] dictatorship based on biblical principles in a land they now call Gilead. Women may no longer possess jobs, or property, or money of any kind. Pollution has sharply reduced fertility, and certain women, selected for their ability to breed, have become slaves - Handmaids - forced to try to conceive through joyless copulation in bizarre menages a trois [sic] with their Commanders and the Commanders' barren wives.As an agnostic who has been terrified for years about the rise in political power of US Christians trying to dictate the details of my life per their Good Book, I can't recommend this novel too highly. Atwood is somehow able to give voice to the very real temptations of humanity, to the darker parts of our psychology that allow bad things to happen, to the fears that paralyze me when I try to verbalize them.
I am personally convinced that America is on the path to becoming a Christian theocracy and that, if we don't stop it in time, the only difference between other religious theocracies and us will be the details of the government-enforced dogma.
Atwood gave specific voice to my fears later in the NYTimes interview (emphasis mine):
I delayed writing it [the book] for about three years after I got the idea because I felt it was too crazy. Then ... I started noticing that a lot of the things I thought I was more or less making up were now happening, and indeed more of them have happened since the publication of the book. There is a sect now, a Catholic charismatic spinoff sect, which calls the women handmaids. They don't go in for polygamy of this kind [as in the book] but they do threaten the handmaids according to the biblical verse I use in the book - sit down and shut up.
... You could say it's [the book is] a response to 'it can't happen here.' When they say 'it can't happen here,' what they usually mean is Iran can't happen here, Czechoslovakia can't happen here. And they're right, because this isn't there. But what could happen here?
It wouldn't be some people saying, 'Hi, folks, we're Communists and we're going to be your new Government.' But if you were going to do it, what would you do? What emotions would you appeal to? What groups would you utilize? How exactly would you go about it? Well, something like the way the religious right is doing things.
And the ultimate result of that process would be the union of church and state, which this country since 1776 has striven to keep apart, with great difficulty, because the foundation of this country was not separation of church and state.
We're often taught in schools that the Puritans came to America for religious freedom. Nonsense. They came to establish their own regime, where they could persecute people to their heart's content just the way they themselves had been persecuted.
If you think you have the Word and the right way, that's the only thing you can do.
The book is an instructional and cautionary tale not just for atheists and agnostics, but for all religious people who honestly can't understand why the atheists and agnostics have become as aggressively anti-religion as they have. This book explains the reasons behind the rise of The New Atheism (see the works of Dawkins, Dennett, Harris, and Hitchens) in response to undercurrents nee waves (soon to be a tsunami) in recent modern America.