"That is why an uneducated believer like Bunyan was able to write a book that has astonished the whole world."
John Bunyan, author of The Pilgrim's Progress
Public DomainJohn Bunyan, author of The Pilgrim's Progress

Lewis is referring, of course, to John Bunyan (1628-88) and The Pilgrim's Progress. Bunyan had, at best, a grammar school education and said of himself, "My descent was of a low and inconsiderable generation, my father's house being of that rank that is meanest and most despised of all the families of the land" (wikipedia).

Bunyan, a Baptist, wrote the allegorical work while serving years in jail for continuing to hold religious services when it was illegal to do so without the approval of the Church of England. Published in 1678, The Pilgrim's Progress has never been out of print since (wikipedia).