Jim Casy is a kind of Christ figure in John Steinbeck's The Grapes of Wrath; though he was once a "real" preacher, he is closest to living out his faith when he gives that up and starts living to serve others. He comes to believe that
"maybe it's all men an' all women we love; maybe that's the Holy spirit--the whole shebang. Maybe all people got one big soul ever'body's a part of."
He lives that out in several ways. As he travels west with the Joads, Casy is always the first to sacrifice his share or to do more than his share out of gratitude to the family. Later he has a chance to sacrifice himself by getting arrested in Tom's place (Christ analogy); while in prison, he learns that the voice of many is stronger than the voice of one as he and the other prisoners protest the quality of their food.
While in prison, Casy realizes his calling is to help coalesce the migrant workers into unions which will make their voices stronger; it will cost him his life, but he is willing to lose his life if that is what it takes.
"Maybe there ain't no sin and there ain't no virtue, they's just what people does. Some things folks do is nice and some ain't so nice, and that's all any man's got a right to say."
Tom Joad certainly does not start out as a man who emulates Jim Casy in any way. He has been in prison and rather feels as if he is owed something in return for his loss of time and freedom. He does care deeply for his family, however, and he breaks the terms of his parole in order to help them travel west.
As he sees and experiences the injustices, his mind is softened to the plight of others. When he meets Casey again after Casy is released from prison, he is not particularly moved by Casy's ideas for organizing the workers. Over time, though, he begins to see the value of such an effort. When Casy is killed, Joad kills the man who murdered Casy and has to live in hiding. This is his thinking time.
When he tells his mother what he feels he has to do, she reminds him that Casy lost his life in this effort, and he may, too. Joad says:
Then it don't matter. I'll be all around in the dark - I'll be everywhere. Wherever you can look - wherever there's a fight, so hungry people can eat, I'll be there. Wherever there's a cop beatin' up a guy, I'll be there. I'll be in the way guys yell when they're mad. I'll be in the way kids laugh when they're hungry and they know supper's ready, and when the people are eatin' the stuff they raise and livin' in the houses they build - I'll be there, too.
Tom Joad has begun to understand, like Casy did, that the souls of all mankind are connected and he must do what he can to make life better for everyone. He has demonstrated Casey's beliefs by how he sacrifices for his own family, and he has spent a lot of time thinking about Casy's beliefs. And now he will take action by actively working for unions among the workers. This is his grand act of sacrifice for a cause greater than himself.