Jim Casy exists as the philosopher, the motivator and the voice of reason in The Grapes of Wrath by John Steinbeck. The ex-preacher is used to express some of the book's major themes, explicitly articulated in his actions. Jim Casy, by fulfilling his predominant role as the novel's guiding moral voice, establishes not only a sense of god, but also one of morality and justice.
Jim Casy is an ex-preacher who is unsure of how to use the talents he possessed as a preacher, if not as the leader to a flock of Christians. He is a fluent and persuasive speaker and spiritual healer but no longer appreciates his talents. By the end of the novel he learns to apply them towards organizing the migrant workers. He comes to believe so deeply in his goal of saving the tormented workers that he gives his life for them willingly.
The ex-preacher is different from the other characters in that he sees his purpose and what is needed to be done much sooner than the other characters.
He sees that in order to overcome their troubles and improve their position, the migrant workers need to mobilize. Casy wants to help the poor and downtrodden because he is one of them himself. He becomes a man full of radical, controversial, 'red' ideas. He becomes a champion of the poor and oppressed and in the end a martyr of his cause.
Early in the book, the Casy is persuaded to say grace over breakfast. After hesitating, he eventually offers these words:
"I got thinkin' how we was holy when we was one thing, an' mankin' was holy when it was one thing. An' it on'y god unholy when one mis'able little fella got the bit in his teeth an' run off his own...