A traveling preacher, Jim Casy was "lousy with the spirit" but troubled by the sinful sensuality that seemed to result from being "all full up of Jesus." He leaves preaching and wanders in the wild country, trying to come to terms with his own ideas about God, holiness, and sin. When we first meet him, he is still struggling with these concepts, but is beginning to narrow them down to an earthy interpretation of Emerson's theory of the Oversoul: All souls are just a small portion of a larger soul, this larger soul being the "Holy Sperit
the human sperit." Being part of this holy spirit means accepting all parts of people, thus "there ain't no sin and there ain't no virtue. There's just stuff people do. It's all part of the same thing." Realizing that these ideas will not be accepted in traditional worship, Casy has declared himself no longer a preacher, although he continues to be a speaker and teacher. Specifically, he shares his theories with Tom, who is an impatient, but not unwilling listener. At various points, Casy's teachings reflect the various philosophies of transcendentalism, humanism, socialism, and pragmatism.
Jim Casy is the moral spokesman of the novel and is often considered a Christ-figure. The initials of his name, J.C., are the same as Jesus Christ, and like Christ, he wanders in the wilderness. In Christ-like fashion, Casy sacrifices himself when he turns himself in to save Tom after an altercation with a deputy. Prior to this point in the novel, Jim has been primarily a speaker, more worried about figuring things out than acting on his ideas. His sacrifice for Tom marks the first time that Casy acts. For his sacrifice, Casy in put in jail, where his experiences with the positive effects of group organization lead him to a more complete realization of his beliefs. He leaves jail and begins to put his theories into practice. He dies a martyr's death, paraphrasing Christ's last words ("Forgive them, Father, for they know not what they do") when he cries, "You don' know what you're a-doin." And, like Christ, his teachings are delivered to the rest of the world as the result of this death. Tom, who must be considered Casy's disciple, vows to spread his message as he works toward greater social justice.