How can I write a thesis statement about existentialism in As I Lay Dying?
A thesis statement needs to introduce your main idea and a "controlling" idea to give body and contrast to your essay. Your topic appears to be to find and explain examples of existential themes in As I Lay Dying, and there are lots of them.
A great way to introduce your main thesis would be to first define the term existentialism to make it clear that you understand the task. Then, your main idea could be something about how Faulkner explores these complex questions of being and not being through varying perspectives of one uneducated rural family. Or, you could put the focus more on the characters and write about how their varying perspectives and personalities portray different interpretations of reality and its value and significance.
Your thesis is going to combine this idea about multiple narrative perspectives depicting the characters' active inner lives and take on the events with a more specific related idea to illustrate and support. You could compare and contrast different perspectives or approach the idea from the question of narrative reliability. Here are some existentialist themes related to two of the characters to guide your thinking.
Darl is one who has a particularly reflective inner life and is given to internal monologues, like when he says, "I don't know what I am. I don’t know if I am or not." Here, he is reasoning out a kind of equation for his own existence, connecting his own being in the world to his brother, his brother’s beloved horse, their soon-to-be-dead mother, and the mule wagon that will carry her body home to rest.
Addie is also given a voice to speak with authority about the nature of living and dying, clarifying her understanding of her life's tragic purpose and revealing herself to be the book's most sympathetic and credible narrator. When Addie, the "I" in the title, speaks to demystify through her own experience the existential crises besetting the family and its members, the reader must pay attention. Addie says, "My father said that the reason for living is getting ready to stay dead. I knew at last what he meant." Here, she is suggesting an end-of-life epiphany allowing her to "clean house" and find peace of mind enough to let go without regret. Although the other family members survive to continued existence, they do not find the resolution and integrity that Addie represents in her passing.