How does Tom’s conversation with Ma establish him as a disciple of the Christ-like figure, Casy? How does Tom adopt Casy’s religious convictions?

When Tom does decide to leave the Joad family, he does so with a new purpose that is a combination of political and spiritual belief. He accepts Casy's belief that there is no individual soul, but instead a collective soul of which each person only has a part, and vows to continue Casy's struggle for better treatment of the workers. This is a turning point for Tom. He previously consigned himself to individualist action for himself and his family, but now wishes to work for the common good. Tom now has a mission to spread his good news and,like a Christ decibel, shake the world up until they care for each other.