“Ulan Dhor" or “Ulan Dhor Ends a Dream” from The Dying Earth by Jack Vance

First appeared in The Dying Earth.

Ulan Dhor, nephew of Prince Kandive the Golden, is sent on a mission to collect tablets from the lost city of.Ampridatvir.  Both desire to become more powerful magicians.  They read a scroll about the legend, explaining what had happened to the city:

"Ampridatvir now is lost. My people have forsaken the doctrine of strength and discipline and concern themselves only with superstition and theology. Unending is the bicker: Is Pansiu the excellent principle and Cazdal depraved, or is Cazdal the virtuous god, and Pansiu the essential evil? " 

"These questions are debated with fire and steel."

Ulan Dhor finds the city and encounters two groups--one that will wear green and not gray, and another vice versa.  Ulan Dhor fruitlessly argues that you can wear any color you want.  He discovers that the two groups are unable to see each other (until he unites the tablets).  He meets and falls for Elai, a young lady of the grays.  They fly about in a flying car, grab the tablets, unite them and...

*SPOILERS HERE ON*

...they release the people to see one another so that they attack.

When Ulan Dhor unleashes the knowledge, he didn't realize the high priest of these religions would still be alive (at least the nervous system of it).  The priest thought he'd only be asleep for a generation before the different religions created peace, but it was 5000 years--and not even an adherent did it.  So he's bent on destroying everyone and saving only Ulan Dhor and Elai to repopulate the religion... except neither wants a part of this. Ulan Dhor thrusts his sword at the brain and flees.  He prays to his flying car, that it help him escape.

Although this hasn't been reprinted, this for me was the highlight of the collection--a thought-provoking meditation on perceived differences between peoples of color or religious groups.  Clearly, differences exist, but they do not justify hate.

The author goes beyond just this.  The prophet-god (?) destroys.  Ulan considers it mad, tries to destroy it, and prays to technology instead.  The scroll reading above basically tells the story.  See esp.: "[The believers] concern themselves only with superstition and theology."  Due to the nature of "and," the last two nouns may be conflated.

Note: I do not have to agree with something to admire its aesthetics.  The over-simplification could read as an insult to religions and their adherents.  There's no need to boycott Jack Vance (I don't know his religious affiliation or politics, if any).  Rather, it's better to engage in dialogue.