When Joad runs into Casy, they get into a conversation in chapter 4 that shows Casy to be a thoughtful man.
Casy was once a preacher, but he tells Joad that now the "sperit" is not so strong in him anymore. He has, for example, thought deeply about the contradictions in the Christianity he practiced. For instance, after revival meetings, he would routinely find a girl and have sex with her, which was a sin—and yet he couldn't stop himself from doing it. He also began to wonder why a girl would not be strong enough to withstand the temptation of sex right after a revival meeting, when she was full of the Holy Spirit. This led Casy to thinking that perhaps sex was not such a sin after all.
He also comes to realize that he is more interested in loving his fellow man than Jesus: Jesus is just an abstract concept to him. So Casy decides that perhaps he is better off ditching his religion and simply devoting himself to loving others. He thinks hard about this and decides that the human spirit is the same as the Holy Spirit.
As a result of his many reflections, Casy turns into a secular person devoted to doing good.