In Stephen Vincent Benét's short story "By the Waters of Bablyon," after John the narrator returns from the Dead Place, what does it signify that John thinks, "Perhaps, in the old days, they ate knowledge too fast"?
Stephen Vincent Benét's short story "By the Waters of Bablyon" was actually written two years before World War II began, just after all of the destruction of World War I. It is a prophetic story in which Benét predicts humankind's fatal abilities long before any nuclear bombs were dropped on Hiroshima and Nagasaki. In the story, the narrator John journeys to a destroyed city his people call the "Dead Place" and reaches a full understanding of just exactly what the people did who had lived there to make it dead.
Once in the Dead Place, John sees that the people had had the ability to, as the editors of the Comprehensive Guide to Short Stories, Critical Edition phrase it, "harness electricity and nuclear power, build subways and elevators, drive cars,fly planes, and create washers and driers, electric stoves, and refrigerators," yet had also destroyed themselves in a "Great Burning" in which fire fell from the sky emitting a "mist that poisoned" (eNotes, "Themes and Meanings"; "By the Waters of Babylon"). We can equate the fire that fell from the sky to bombs, even a great, fatalistic bomb, which predicts the upcoming nuclear bombs.
Essentially, the John sees that the destruction of the city was the peoples' own doing; they had acquired so much knowledge that could have been used for their own good but instead used it to cause their own destruction. Hence, when John says towards the end of the story, "--it is better the truth should come out little by little. I have learned that, being a priest. Perhaps in the old days, they ate knowledge too fast," he is warning against how destructive knowledge can be, especially when it's gained too quickly. When we gain knowledge too fast without taking time to stop and think about all its ramifications and how best to use it, knowledge becomes destructive rather than beneficial.