It is unknown if the cult existed before Eidolon's invasion of the world or emerged later, after Famine had already taken control of the continent. In the latter case, the probable purpose of the cult is to appease or gain the favor of the dreaded Rider of the Apocalypse.
They built a secret chapel with an alter in the town's Shoppe, suggesting that its owner was a member of the brotherhood. The chapel could only be entered by a person who wears the Amulet of Hunger - a symbol of membership in the cult.
Reading the book on the altar reveals the following passage:
"And it is foretold that by seeking out sand, glass, and grindstone, one may cross the crystal barrier and join with the Master of Hunger. Your quest begins at the fireplace by the forge."