Everyone Is Jesus in Purgatory: Steinbeck stated that there are five levels to the story. Presumably, one is the tale of the Joads themselves, one is the Depression, one is man's inhumanity to man in general, one is why there are so many biblical references, and one . . . Who knows?
Also there's the uncanny resemblance between Jim Casy and Jesus Christ.
Harsher in Hindsight: The book has many themes that are still relevant today. Especially for the U.S. which suffered a heavy recession in 2008 and is suffering from water shortage and other agricultural problems that are making lives difficult for many. It's arguably because of this that the book is still banned in many schools and other places to this day.