What does Tom Joad tell his mother when she asks him how she'll know what happens to him (that is, whether he's hurt or killed after he leaves them)?

As I recall from the movie, Tom talks about his spirit always being there when someone is mistreated or exploited because they are poor. I don't recall the exact words but his point is that he will fight against injustice. Tom becomes a Christ-figure who is willing to sacrifice himself for the good of the oppressed.