Who is Reverend Casy? Well, he's a lecherous old man who has given up on his life as a preacher. That's the simple answer. But, boy oh boy, do we miss out on a lot of good stuff if we forget to look further. Casy also happens to be the spiritual compass of the Joad family and of the entire novel.
Casy trumpets a kind of philosophy similar to that first drummed up by the famous American philosopher and thinker, Ralph Waldo Emerson. Emerson believed in one giant, invisible, collective soul that contains the souls of all the creatures of the world. He called this the "Over-Soul." (Read Emerson's essay on the Over-Soul here.) Reverend Casy hold a similar belief. Casy even tells Tom, "Maybe all men got one big soul ever'body's a part of" (4.41). Folks, we have the privilege of witnessing a man develop his own philosophy, piece by piece. And just where does this philosophy lead him? Toward a journey westward where he hopes to help his fellow workers. And where does this journey lead him? Toward a mission to fight for equality and justice among migrant workers in California. And where does this mission lead him? Toward his death.
In this light, Reverend Casy is a martyr, he's basically killed because of his beliefs. When we think long and hard about this preacher's life – how he disappeared from Sallisaw for a while and wandered around, how he loves people and being among people so much – we realize that he reminds us of someone. He reminds us of another martyr who wandered in a wilderness for a while, developing his own philosophy, and who loved people so much. He reminds us of Jesus Christ. And wouldn't you know, Jim Casy shares the same initials as Jesus Christ – J.C.
Tom Joad encounters Casy on his way home from jail. Casy sits under a tree and talks on and on about all of the times he slept with women when he was a practicing preacher. He has some serious guilt about this, and believe us, this guilt does not go away. At the same time, however, Casy is drawn to life and to the people who live life. While he was a preacher, Casy was put on a pedestal and was distanced from the people around him. He doesn't want to be isolated from mankind ever again. It's too lonely and too unnatural. Casy has realized that chilling with people what life is all about.
There's something a little troubling about Casy. He's always thinking, and he is always just a wee bit quiet. Casy is constantly digesting the world around him, and he reveres people and the small details that go into daily life. He becomes a kind of mentor to Tom Joad throughout the novel, and the two men seem to understand the injustice of the world around them better than anyone else. Casy constantly articulates his belief in a giant collective soul that connects people, and Tom grows to believe in this philosophy as well. Casy finds purpose in his life, and, as a result, he inspires Tom. When Casy dies fighting for the rights of migrant workers, Tom takes up the fight.