What is the theme in The Life You Save May Be Your Own by Flannery O'Connor?

Search for the meaning of life is the main theme. When Shiftlet approaches the Crater's farm, it is not clear what type of person he is. What is apparent is that he is searching for something. By marrying Lucynell and then abandoning her, he has missed an opportunity to experience redemption (an event symbolized by the "guffawing peal of thunder" and his anguished plea to God at the end of the story). Shiftlet has failed to bring meaning into his transient life. He entered the Craters's lives as a lonely wanderer, and he leaves it the same way.