A theme of “The Life You Save Might Be Your Own” is focus on God versus material things.
At the beginning of the story, Mr. Shiftlet is a Jesus-like figure. He is a tramp carpenter who passes by one day and talks philosophically with the “old woman” Mrs. Crater. He is not interested in money or material possessions.
"Lady," he said in a firm nasal voice, "I'd give a fortune to live where I could see me a sun do that every evening."
Yet as he remains on her farm, Mrs. Carter corrupts him. He first stays in an old car and does odd jobs for her for no money, but then he fixes the car. He accepts money to marry her. He becomes more interested in the material, and less interested in the spiritual.
The difference in Mr. Shiftlet’s character is especially evident when he picks up the hitchhiker. He tells the boy how he has the best mother in the world, and it infuriates him such that he jumps out of the car. As Mr. Shiftlet drives on, he realizes he has turned his back on God. The sun he spoke of at the beginning of the story is covered with a cloud, and the rain hits his car.