As the truck returns to the highway, Tom walks down the road toward his family's farm. The hot sun beats down on him, so he takes off his shoes and wraps them in his coat. Spying the horned turtle from the previous chapter, he picks it up and wraps it in the coat as well. Continuing down the road, he sees a man lying under the tree, singing to Jesus. Tom recognizes him as the preacher, Jim Casy, but Casy is quick to tell him that he has been filled with sinful thoughts and is no longer a preacher.
Bothered by his need to have sex with a young girl after a meeting, Casy has been wandering about, trying to figure out how men can be "sinful" when they are full of the Holy Spirit. He has lost faith in organized religion, finally deciding that what it's really all about is love: not love of Jesus or God, but love of all men. He has come to the conclusion that no one has an individual soul, but that everyone's soul is a part of a larger soul that includes all people. With thoughts like these, he feels he should no longer be a preacher. Tom agrees, and Casy decides to walk to the Joad farm with Tom. When they reach the Joad place, it is deserted, and Tom realizes that something is wrong.
Of critical importance to the novel, Chapter 4 provides the first strand of a social philosophy advocated by Steinbeck: an evocation of the Emersonian concept of the Oversoul. This idea is delivered by the character of Jim Casy, who is believed to be the carrier of Steinbeck's philosophical beliefs. When we first meet Casy, we learn that his ideas of religion and spirituality have changed. Troubled by his own sensuality, and wrestling with the concept of sin and virtue, he has "gone off on his own to give her a damn good thinkin' about." When he returns, he has experienced a re-birth, a re-consideration of the Holy Spirit and what it means to be holy. Casy has decided that sin and virtue are all part of the same thing. The souls of all humans are only small parts of a larger soul that encompasses everyone — the Oversoul. All that really matters is love of all men and all women, and the Holy Spirit is, in fact, the human spirit. Humans are what Casy loves, not this person he does not know named Jesus. He is turning from an abstract concept to a more personalized form of religion based on the actions of individuals.
The structure of the novel shows the general plight of the "Okies" by focusing on the specific problems of a single family. Connections are constantly maintained between the general, or intercalary, chapters and the narrative chapters. In this case, Tom picks up the turtle from the previous chapter. This correlation between the abstract and the specific is also characterized by the contrast between Jim Casy and Tom Joad. Casy deals with the theoretical, concerned with defining the problems that are facing humanity. Although he has abandoned a religion of general ideals, it isn't until much later in the novel that he physically supports his beliefs through action. Tom, on the other hand, is a man of action, although his motivation is primarily self-centered. He is concerned with himself and his own family, but eventually grows because of his intuitive response to people in need. In the end, abstract thoughts are not what matter as much as the actions of individuals.
meetin' an assembly or place of assembly for worship.
prodigal here refers to the wastrel son in biblical scripture who was welcomed back warmly on his homecoming in repentance (Luke 15:11-32).