When approaching an assignment such as this, specifically one that specifies that you use 500 words, concise concepts will be important. This demands the utmost clarity in terms of your thesis statement. Luckily, the prompt is incredibly clear.
In the short story referenced in your question, specifically "Salvation" by Langston Hughes, the topic of faith is the most central of themes. The idea that separate people have different ideas for the concept of faith is actually the central conundrum in the story. Young Langston has been told his entire life that he is going to be saved and "see Jesus." Older members of the church all around him have testified to the physical sensation of being born again.
However, when the ceremony to save the "little lambs" is underway, young Langston finds himself waiting to literally see Jesus. He passes through many levels of disappointment, such as thinking that there is something defective within himself and realizing that the adults had perhaps been exaggerating or speaking metaphorically. At the end of the night, he cries, because he eventually just pretended to be saved and could not believe in Jesus because he could not see Him.
These are all things to consider when writing your thesis about the story. Most importantly, you might contrast the adults' definition of faith with young Langston's and how it alters his understanding of faith forever. One example might be as follows:
In "Salvation," Langston Hughes explores a child's exposure to the social aspects and pressures of organized faith that, while common in adulthood, can permanently damage a young child's views on religion.