In multiple works they have used the idea that old gods still exist but are weaker or fading due to lack of belief.
Off the top of my head I can think of a few that fit. *American Gods by Neil Gaiman and the Who Mourns for Adonais?*episode of Star Trek TOS. I haven’t caught up with all the books yet but the Dresden Files uses this.
There are other examples that are just beyond the grasp of my memory. So what are some other examples and who did it first?
Oops. Reported my own post for the move to CS.
Larry Niven’s The Magic Goes Away had this.
Lots of choices here. Two off the top of my head.
It’s a minor but increasingly important element of George RR Martin’s Song of Ice and Fire stuff.
It’s the main focus of Small Gods by Terry Pratchett.
Moving from IMHO to Mount Olympus, er, Cafe Society.
Puck of Pook’s Hill by Kipling has something along those lines with the Wayland Smith arriving in England and slowly fading in power and stature as his believers disappear.
Puck however remains as he always has.
And the reverse is a small plot point in Pratchett’s Going Postal. It involves the rise of the goddess Anoia, Goddess of Things Stuck In Drawers.
Turtledove’s Case of the Toxic Spell Dump exists in a world that relies on all religions, prior gods etc. for its tech and engineering. The plot revolves around clashing religions and god-sets and the artificial preservation of things like the Cult of Apollo.
It also ruthlessly Anglicizes all place-names in the southwestern US to absolutely devastatingly funny effect. Highly recommended.
Man, I haven’t thought of that book in a while. I think I have a copy laying around. Probably my favorite Turtledove novel.
“Night Life of the Gods” (1931) by Thorne Smith, an author best known for the “Topper” series of books.
In NLOTG, an inventor creates a 1920s style transmogrifier ray that turns people into stone, and vice versa. (Dr. Frank’n’Furter ripped off his idea about 30 years later.) Someone aims the ray at statues of the Roman Gods in the NYC Metropolitan Museum of Art and turns them into real “people.” The whole lot of them go on merry adventures in prohibition era NYC.
Weird. I just yesterday tossed that paperback into the car for reading at meetings. Great book!
There was a miniseries on NBC called Merlin that featured Mab, a pagan goddess on the verge of extinction because she was being forgotten after the spread of Christianity.
It’s been a while since I read it, but IIRC Tom Robbins’ novel “Jitterbug Perfume” features Pan, who has become invisible due to lack of belief. However, even if you can’t see him, you can definitely smell him.
Anderson’s A Midsummer Tempest fits, I think. Faeries and Herne and all that.
Adams’ The Long, Dark Tea-time of the Soul; though I don’t remember if the gods were weak there or just bored and tired.
Ditto ditto Tom Holt’s Expecting Someone Taller (I think it was), where an ordinary Joe gets ahold of the Ring of the Niebelung (or however it is spelled) and with the (largely subconscious) power of life and death over the whole world …
everything actually goes quite well, much to the dismay of the world’s news broadcasters.
Ha! I remember that book. It was one of my favorites for a while, but I haven’t read it in twenty-mmmrmr-years.
Piers Anthony uses this idea in the Incarnations of Immortality series. For example, the incarnation of good that we would call God is said to be a relatively new diety and very powerful because of strong belief. The Jewish god is shown as a separate person, with diminished power due to having less followers and emaciated during the holocaust.
Several of Holt’s books deal with stuff like that: Ye Gods, Here Comes the Sun, Odds & Gods, and such.
Pan was all over British literature for much of the 1890’s. By the end of the decade, one noted wit made fun of the endless stream of horned li’l gods popping out the forest “to wean quiet English villages from respectability.”
Arthur Machen’s The Great God Pan - an excellent example - inspired Lovecraft.
Hence the user name? 
(Wasn’t Ellen Cherry one of the waitresses in that book?)