How can Jim Casy be understood as a Christ-like figure?

The character of Jim Casy moves from being an energetic preacher to shunning God. He then becomes something of an apostle. Finally, as his initials "J.C." suggest, the character becomes Christ-like.
The home everyone once knew in this rural section of Oklahoma is gone. For the few people who remain, the need for a leader grows stronger. Casy, like the Apostle Paul, reluctantly assumes the mantle of leadership. While he can never again embrace the former evangelical role many in his community expect from him, Jim Casy becomes more and more Christ-like until eventually, like Christ, Casy dies a martyr’s death.
As the novel opens, Tom Joad finds the former preacher resting under a tree. Tom does not recognize the old family friend at first; Casy reminds Tom how they know one another
"I was a preacher," said the man seriously. "Reverend Jim Casy—was a Burning Busher. Used to howl out the name of Jesus to glory. And used to get an irrigation ditch so squirmin' full of repented sinners half of 'em like to drowned. But not no more," he sighed. "Jus' Jim Casy now. Ain't got the call no more. Got a lot of sinful idears—but they seem kinda sensible."
Despite Casy’s disillusionment, evidence of his good moral character is presented early on. Casy listens patiently to Muley Graves, the lone resident of the now-deserted land that Tom had once called home. Muley tells Tom and Casy about the plight of the people, of the disrespect for their land,their families, and their homes. The injustice of it all ignites passion in Casy, perhaps really for the first time. It is as if he is experiencing his own private Pentecost:
Jim Casy had been staring at the dying fire, and his eyes had grown wider and his neck muscles stood higher. Suddenly he cried, “I got her! If ever a man got a dos of the sperit, I got her. Got her all of a flash!” He jumped to his feet and paced back and forth, his head swinging. “Had a tent one time. Drawed as much as five hundred people ever’ night. That’s before either you fellas seen me.”
Because the reader knows Casy, unlike a lot of traveling preachers, has no monetary or self-aggrandizing motives, it is easier to trust him as the landscape, and the family, changes.
Casy's development as an apostle continues as the family prepares for their frightful journey into the unknown. When it is time to head out, and it is decided that Casy will accompany the family, Ma asks him to say a prayer. To the surprise of all who had known him, this is not the fire-and-brimstone preacher of the past, but the new, introspective man of spiritual matters. As the family bows their heads, Casy tells the assembled Joads:
“I been thinkin’, “ he said. “I been in the hills, thinkin’, almost you might say like Jesus went into the wilderness to think His way out of a mess of troubles…
Seems like Jesus got all messed up with troubles, and he couldn’t figure nothin’ out, an’ He got to feelin’ what the hell good is it all, an’ what’s the use fightin’ and figurin’. Got tired, got good and tired, an’ His sperit all wore out. Jus’ about to come to the conclusion, the hell with it. An’ so he went into the wilderness. . . .
I ain’t sayin’ I’m like Jesus,” the preacher went on. “But I got tired like Him, an ‘ I got mixed up like Him, an’ I went into the wilderness like HIm, without no campin’ stuff. Nighttime I’d lay on my back an’ look up at the stars; morning I’d set an ‘ watch the sun come up…”
While Casy is not arrogant enough to believe himself on par with Christ, he does recognize that perhaps, like the apostles, he possesses a gift for prophecy. When Casy reflects on the fate of Muley Graves, he sadly remarks,
“Course Muley's crazy, all right. Creepin' aroun' like a coyote; that's boun' to make him crazy. He'll kill somebody purty soon an' they'll run him down with dogs. I can see it like a prophecy."
Casy continues to be a spiritual touchstone throughout the novel. Initially, Casy feels much like the Apostle Paul in the Bible. Paul is frequently called the “Reluctant Apostle” because he does not feel worthy of the task given to him by God, that of leading the people away from everything they have known into unfamiliar territory. Like Paul, Casy has initial feelings of helplessness and unworthiness. He worriedly tells Tom,
“I got the call to lead people, an’ no place to lead them.”
But eventually, Casy comes to accept his role. In the final moment of his life, Casy calls out:
"Listen," he said. "You fellas don' know what you're doin'. You're helpin' to starve kids. . . .You don' know what you're a-doin'."
Casy has lived up to his initials, J.C. as his final word echo those of Christ himself, who, as he is being nailed to the cross, cries out, “Forgive them, Father, they know not what they do,” (Luke 23:34).