There are several different themes in The Grapes of Wrath, by John Steinbeck. Here I will go into depth on those. The three main themes in the story are free will versus necessity, the holiness of every man, and the kinship of all man.

The main theme (and the most important, in my opinion) is free will versus necessity. All throughout the story, the characters are forced to do something either because they want to or they have to. A good example would be Ma's burning her old souvenirs when they leave for California. She doesn't want to do it (free will), but she knows she must (necessity), as they don't have enough room to take them with her. Another example of this would be Jim Casy's sacrifice of himself in place of Tom for tripping the police officer. He had been thinking of how he could repay the Joads for their kindness (he was willing, that is the free will part). When he sees the opportunity present himself, he feels compelled to do help (necessity) by accepting the blame.

Another important theme in the story is the holiness of every man. This is what Casy came to believe after he had spent a great amount of time thinking about it. He came to believe that every man was holy, and so, he didn't need to be preached to, since he was already holy. He began to think of everyone's soul coming from the Oversoul when they were born, and their soul going back to the Oversoul when they died. This is one of the reasons that Jim Casy was so generous; his soul was already a part of the Oversoul, so he might as well go ahead and do good deeds (although, I'm not sure that that was exactly what he thought).

All throughout the book, the theme of kinship of all man is reenforced. The journey to California is supposed to be just the family. Then, they are joined by Jim Casy. After a while, they join the Wilson family. After the they lose a few family members and arrive at the camp, Ma sets out a little bit of food for some hungry children, even though her own family barely has enough to eat. During the flood in the final chapters, the Joads and the Wainwrights become practically one family; not only are two members getting married, but the close conditions force them to simply accept each other's being there like they would a family member (I don't think I'm making much sense anymore...).