Perhaps, the main question that arises from the narrative of John Steinbeck's seminal work, The Grapes of Wrath, is an existential question: Can man exist as a singular being?
Early in the novel, Tom Joad makes his way home from prison; along the way he meets the former preacher named Jim Casy, who has lost his faith. But, as he talks with Tom, he reasons,
"...maybe it's all men an' all women we love; maybe that's the Holy Sperit--the human sperit--the whole shebang. Maybe all men got one big soul ever'body's a part of...."
Casy’s philosophy is thus constructed, much like that of Steinbeck, upon the concept that all human beings are existentially linked in love and charity to one another. When Casy dies, Tom assumes this philosophy of the Oversoul, promising to take up Casy’s mission of unifying the disenfranchised and displaced migrants. Before he departs, Tom tells Ma not to worry about him, saying,
"I’ll be ever’where—wherever you look. Wherever they’s a fight so hungry people can at, I’ll be there. Wherever they’s a cop beatin’ up a guy, I’ll be there."
This idea of the unity and fraternity of man as the issuance of strength against poverty and starvation grew in the minds of many from the Depression era and it was also one that the naturalist Steinbeck embraced, as well.
The Grapes of Wrath is a very long book that offers many questions. So it is difficult to pinpoint a single question, and to have everyone agree on what it is. In light of this, let me offer a few questions that are central to the book.
First, the most obvious question is whether the Joads will make it. They struggle from the beginning of the story to the end. They leave Oklahoma to sojourn to California, because they hear of job opportunities. The question within their struggles is will they make it there (to the Promised Land). They do.
Second, as the story progresses, the other question that begins to arise is whether they will stay together and do something that will help the migrant works in general. At the end of the story, it becomes clearer that the Joads are a special stock of people. They will not only help themselves but others. For example, Rose of Sharon will feed a dying man her milk and Tom will organize others to fight injustice.