The theme of "Gimpel the Fool" is the power of belief and the nature of faith. Gimpel is a character who believes everything he is told. The other people in his village think they are deceiving him and treat him as a laughingstock, but Gimpel prefers to think the best of everyone and give them the benefit of the doubt. The best example of this is his marriage to Elka, who repeatedly cheats on him and has many illegitimate children that she convinces Gimpel are his. Even when Gimpel catches other men in her bed, she is able to convince him that he has been seeing things.
In fact, Gimpel is not a fool at all. He knows that Elka is deceiving him; he even goes so far as to separate from her. What he comes to understand, however, is that the power of his belief and love is more important than the "truth" about Elka's infidelities.
There are numerous themes in the the short story "Gimpel the Fool," but the most prominent one is probably the difference between perception and reality. Gimpel struggles constantly with what is real and what is illusory in the story, being mocked for his prevailing belief in God and the afterlife (in spite of the futility of doing so according to the other characters) and living essentially in a fantasy world.
His perception also impacts his relationships and the life he leads. When he marries, he is convinced by his unfaithful wife that all of her eventual children (including the one with whom she is already 5 months pregnant) are all his, in spite of obvious evidence to the contrary. His foolish beliefs make him the butt of many jokes, but they also make him the more optimistic and kind character throughout much of the story.
In a very broad sense, a prominent theme of "Gimpel the Fool" is trying to reconcile one's beliefs with the truth of world around them (beliefs vs. reality).
On the surface, "Gimpel the Fool" tells readers a tale of what seems to be a very foolish man, one who is disrespected by his wife, his community, and employees. Yet through all of Gimpel's misfortunes, he retains belief in God and in the truth that could potentially be on other's lips ("In the first place, everything is possible, as it is written in the Wisdom of the Fathers," Gimpel the Fool, p. 1). Gimpel's beliefs frequently cause him to be made a fool of in the reality of his social sphere, although Gimpel's ending epiphany informs readers that his beliefs triumph: "No doubt the world is the imaginary world, but it is only once removed from the true world" (Gimpel the Fool, p. 10).
Thinking more symbolically, the theme of beliefs vs. reality is realized when you read the short story as a symbol of the Jewish experience post-WWII. Gimpel and the people of Frampol are Jews themselves, struggling with what seems real in the natural world and what is true in the spiritual realm. When this story was written in 1953, many Jews had been displaced and affected by the horrors of the Holocaust. Many could find pieces of themselves in Gimpel's story; they, like Gimpel, were asked to hold on to the "truth" that they were still the chosen people of God despite the trauma they as a whole had gone through.