"By the Water of Babylon" is a warning against using human knowledge too quickly without thinking of the consequences. At the end of the story, John says that the people in the City of the Gods, "ate knowledge too fast." In other words, they developed weapons and then used those weapons before they fully understood what the consequences for their civilization would be. Many people assume that the author was warning against the atomic bomb. However, the story was published years before the atomic bomb was first detonated, so his warning is probably much broader and includes other weapons of mass destruction such as chemical and biological weapons which did exist at the time he wrote this story. The author may also be warning about an inherent flaw in mankind. When John returns to his people, he vows to learn the knowledge he found in the City of the Gods. He is sure that he can use the knowledge wisely. His father, who is older and wiser cautions against this, but John, in his youthful enthusiasm, plans on ignoring his father's advice and teaching his people the new knowledge. Thus, the cycle of destruction may simply continue.