Engine Summer
From back cover Bantam paperback March 1980:

RUSH THAT SPEAKS

Born into the community of Truthful Speakers one thousand years after the Storm, he was raised on stories of the old days -- a world filled with saints, a world in which all things were possible, a world which finally destroyed itself.  In love with a beautiful woman, Rush journeys far and learns much.  Taken into the society of Dr. Boots's List, attached to the old mysteries, Rush grows closer to a sainthood he could never have imagined.
Science fiction
John Crowley