In order to understand Jim Casy's philosophy of life, we have to first know that he was a preacher in his former life. He may have left the ministry (because he had sinful thoughts, as he said), but aspects of Christianity stayed with him, which shaped his philosophy.
He did not like the ethereal aspect of Christianity that emphasized the otherworld, because it did not emphasize the needs in the present world. He refined what it means to be a Christian or a holy person by saying that what is most important is found on earth among people, that is, to be with people and to serve and help them.
For this reason, he used his gifts to organize people and create a movement that would help people in the hardships that they face. This was such a central philosophy that he paid the ultimate price of his life as a martyr.
It is easy to see that J.C., his initials, are the same as those of Jesus Christ. He is the Christ-figure in the book, but an earthly one.