Orphans of the Sky by Robert A. Heinlein

The Jordan Foundation sponsored the Proxima Centauri Expedition in 2119, in attempt to reach the nearer stars of the galaxy. But that was far in the mythic past.

The original purpose of the Ship epic voyage has long been forgotten, and gor generations the ginat spaceship, lost between the stars, is the only world that the people aboard have known. A strange civilization has evolved, with its own superstitions, savage religion, rigid class structure and mutant outcasts. Then, one young man discovers the truth about the Ship and changes everything, for ever…