So another book down on my rereading-Heinlein project. And only 27 left to go!
This 1941 short novel was originally titled Sixth Column, and you can buy it from Amazon under that title. (I just reread the original, beat-up, 50¢ Signet paperback I got back in the 1960s.) The premise is that the USA has been taken over (very easily) by the "Pan-Asians". The only remnant is a super-secret "Citadel" in the mountains, a research lab in charge of developing weaponry at the cutting edge of physics.
And they've succeeded. Just a little too late to be of any help in deterring the Pan-Asian invasion. There's only six of them left, too. Because testing their latest gadget killed nearly everyone else in the facility.
So the survivors face a problem: even though they have this nifty new discovery (and it has a lot of other uses besides indiscriminately killing people), it's pretty clear that there's no obvious strategy that will get the country back. Sheer numbers of the ruthless Pan-Asian hordes preclude any straightforward attack.
Unless… hey: the Pan-Asians are pretty tolerant of one thing only: the religion of the conquered masses. So the good guys come up with a fake religion, meant to disguise recruitment and deployment of their forces and weaponry across the country. (There are some omens of Stranger in a Strange Land in the discussion of religion design.)
Complicating things: the chief scientist at the Citadel is, well, the worst kind of scientist. A constant thorn in the others' sides, and (at the climax, spoiler, sorry) a genuine threat.
I was kind of kidding when I said the nifty new discovery killed people indiscriminately. In fact, it can be set to discriminate. Specifically, it's a death ray that can be tuned to only kill a certain race? Now, there's a thorny ethical problem! It's arguable that Heinlein dealt with this in as enlightened a manner as possible, given the era in which it was written. But I don't see this book being assigned to readers in your local schools and colleges without a major fuss.