The mezuzah is mentioned only one time in the short story "Gimpel the Fool" by Isaac Bashevis Singer. Its significance is best understood in relation to the crucial transformation Gimpel undergoes that causes him to kiss the mezuzah.
The story tells of a man named Gimpel who is derided as a fool by everyone in the village of Frampol because, in his innocent naiveté, he believes everything that anyone tells him. The rabbi whom Gimpel approaches for advice tells him a profound truth that is one of the story's main themes:
It is written, better to be a fool all your days than for one hour to be evil. You are not a fool. They are the fools. For he who causes his neighbor to feel shame loses Paradise himself.
Nevertheless, the villagers continue to mock Gimpel. They talk him into marrying a woman named Elka, claiming she is a virgin when in fact she is a widow and a divorcee. She has six children, none of whom are Gimpel's. Even when Gimpel catches her in bed with other men, she somehow persuades him that he is seeing things. Gimpel's good-hearted nature allows this deception. He chooses to maintain his integrity despite all the evidence that contradicts his simpleminded view of his loved ones and the villagers. After the birth of one of his children he reflects:
All Frampol refreshed its spirits because of my trouble and grief. However, I resolved that I would always believe what I was told. What's the good of not believing? Today it's your wife you don't believe; tomorrow it's God himself you won't take stock in.
Near the end of the story, Gimpel's wife gets sick. Before she dies, she confesses the truth to him. She says, "It was ugly how I deceived you all these years. I want to go clean to my Maker, and so I have to tell you that the children are not yours." After this, the spirit of evil appears to Gimpel and tempts him to pollute the bread of the village with his urine. Gimpel does this, but then realizes his mistake and destroys the bread. He realizes that he cannot live any longer in the village where he has known so much falsehood, and so he divides up his wealth among his children and takes to the road as a wandering mendicant.
Now we come back to the mezuzah. A mezuzah is a small scroll of parchment containing words from the Torah that is placed in a case and affixed to the doorpost of a Jewish home. This is in fulfillment of a commandment of God in the book of Deuteronomy 6:9, which says concerning the words of God: "And thou shalt write them upon the posts of thy house, and on thy gates."
When Gimpel kisses the mezuzah before he leaves his house forever to wander the land, he is reaffirming his faith and trust in God. We see, then, that the mezuzah is a very good thing, because for Jewish people it represents their faith in God.