The subject of religion plays a significant role in several of the "Mars" (or "Barsoom", the native name for Mars) stories by Edgar Rice Burroughs, and is discussed incidentally on several additional occasions.
The Holy Therns
The Holy Therns are the visible keepers of a cruel religious order which is taken for granted in the first book and largely toppled in the second. More about this at a later time.
The Turian Religion
The religion of Phundal, a city-state on Barsoom, includes a deity named Tur and a holy book called Turgan.
|from "The Master Mind of Mars" (book #6, first published 1927):|
...the Phundalians maintained that Tur still created every living thing with his own hands. They denied vigorously that man possessed the power to reproduce his kind and taught their young that all such belief was vile; and always they hid every evidence of natural procreation, insisting to the death that even those things which they witnessed with their own eyes and experienced with their own bodies in the bringing forth of their young never transpired.
Turgan taught them that Barsoom is flat and they shut their minds to every proof to the contrary. They would not leave Phundahl far for fear of falling off the edge of the world; they would not permit the development of aeronautics because should one of their ships circumnavigate Barsoom it would be a wicked sacrilege in the eyes of Tur who made Barsoom flat.
They would not permit the use of telescopes, for Tur taught them that there was no other world than Barsoom and to look at another world would be heresy; nor would they permit the teaching in their schools of any history of Barsoom that antedated the creation of Barsoom by Tur, though Barsoom has a well authenticated written history that reaches back more than one hundred thousand years; nor would they permit any geography of Barsoom except that which appears in Turgan, nor any scientific researches along biological lines. Turgan is their only text book – if it is not in Turgan, it is a wicked lie.
|from page 112 of the Ballantine paperback edition|