Early in the book, we are introduced to Eusa, a mythologized figure of religious stature. His name is used as an oath, and his character is featured in allegorical puppet shows. Later, when Riddley is captured by Goodparley (about halfway through the story), we learn about the historical significance of St. Eustace, an early Christian saint -- known also by his Roman name Eustachius -- whose legend was painted in Canterbury Cathedral.
Goodparley has equated Eusa with the ancient St. Eustace, and begins to point out the similarities in the Eusa Story -- a parable, written in numbered verse, about the events leading up to the "end of the world" -- and in the information about he has found. His interpretations are based on the little he understands about the old ways (since the criticism he found of the painting dates back thousands of years, probably to a time around the 20th century as we know it), and any attempt at his theories to correctly translate the Eustace myth are erroneous. Instead, he draws comparisons to the history he knows and equates this Eustace with nuclear destruction, much like in the Eusa Story.
Eusa becomes a religious figure both through his relation to Christianity as St. Eustace, and also from his reverence as the creator of the "1 Big One." In this society, the government has regulated the distribution of Eusa worship. The "mincery" is the one who produces the travelling puppet shows ("mincery" being both ministry and office of the Prime Minister; linguistic combinition of government and religion).
But Goodparley isn't completely off track. The Eusa character has Christian theology incoporated into his mythology until they are two heads of the same coin. Eusa heads off into the "Hart uv the Wud" to find the "little shyning man the addom" (or Adam, perhaps?) This "hart" (or stag) he finds in the center of the woods. There he finds, as did St. Eustace, a stag with a cross between its horns, and a small figure on the cross. To St. Eustace, this was Jesus. To Eusa, it was "the little shyning man," whom Goodparley later determines must have something to do with chemistry since he was crucified (like the word "crucible"). Both St. Eustace and Eusa lose their families as a test of their fortitudes, and are eventually reunited when they seem to be humble.
St. Eustace and his family are cooked alive when they refuse to honor Roman emperor Hadrian's pagan gods. Goodparley believes that this "brazing boal" must have something to do with charcoal production and more science. Eusa experiences a parallel end when the Ram chops of his head for refusing to say "trubba not" at their gates (Ram and Rome do sound awfully similar as well). Both men are martyred for their actions, and their stories are told and retold over time.
The Eusa Story is hardly a straight adaptation of the legend of St. Eustace, and it was doubtfully supposed to be. After all, Eustace never doubts or destroys Jesus the way Eusa does of the Little Shyning Man. Also, St. Eustace wandered into the woods while hunting (he is one of the patron saints of hunters), while Eusa was sent there by Mr. Clevver to find a way to create a powerful new weapon. Mr. Clevver was "Big Man uv Inland" and portrayed in the puppet shows by the a puppet with horns and hooves.
Does this mean that Eusa was sent by the devil and eventually destroys Jesus (the "Little Shyning Man") with technology? It's not such a far-fetched idea... when Riddley leaves Goodparley and goes to Cambry ("zero groun" and home of "stoan trees" of the great cathedral), he says:
"Somers in be twean them stoan trees and the Power Ring they musve put ther selfs wrong. Now we dint have either 1." (pg.162)
Maybe in creating so many machines and weapons that led to such destruction, man lost his capacity for religion. Civilization got lost somewhere between the religion and the science, and ended up the technologically barren land that Riddley knows.
