What is the significance of the reoccurring motifs of fire and hunger in "By the Waters of Babylon" by Benet?
Fire is used many times throughout the story. Sometimes John uses fire to do realistic things like cook food or stay warm. At other times John uses the word "fire" to describe a deep burning desire that he has. For example, John explains that acquiring new knowledge was like a fire in his heart. He yearned for more.
My knowledge made me happy—it was like a fire in my heart.
Unfortunately I can't fully support the idea of "hunger" being a reoccurring motif. The word is used only twice in the entire story. However, I think perhaps that the question is asking that "fire and hunger" be thought of as a single concept.
John isn't hungry for food. He's hungry for knowledge. He yearns to know the answers to his questions. That hunger burns in his soul, and he is incapable of resisting the basic burning of his hunger for knowledge. For example, about halfway through the story, John arrives at the Place of the Gods. He knows that he should turn around and go home, but he can't. He is drawn toward the city by some primal need -- a hunger.
Surely, that was enough to do, and live. Surely it was enough to spend the night upon the cliff. The Forest People themselves do not come near. Yet, all through the night, I knew that I should have to cross the river and walk in the places of the gods, although the gods ate me up. My magic did not help me at all and yet there was a fire in my bowels, a fire in my mind. When the sun rose, I thought, "My journey has been clean. Now I will go home from my journey." But, even as I thought so, I knew I could not. If I went to the Place of the Gods, I would surely die, but, if I did not go, I could never be at peace with my spirit again. It is better to lose one's life than one's spirit, if one is a priest and the son of a priest.
The above paragraph mentions that John has a fire burning in two locations. One is his mind, and the other is his bowels. Bowels will be the intestines. Basically anything past the stomach. If the stomach and bowels are empty, the body sends signals to the brain that you understand as hunger. John has a mental and physical hunger to go to the Place of the Gods, and it burns like fire in his soul.