What is the significance of the remarks made by the hitchhiker just before he leaps from the moving car?

Near the end of the story, Mr. Shiflet abandons Lucynell and gets in his car to travel to Mobile, Alabama.  Along the way, he picks up a boy that he thinks is a hitchhiker.  The boy doesn't have his thumb out, but Shiflet picks him up anyway: "I see you want a ride."

Shiflet goes on about his mother:

It's nothing so sweet as a boy's mother. She taught him his first prayers at her knee, she give him love when no other would, she told him what was right and what wasn't, and she seen that he done the right thing. Son, I never rued a day in my life like the one I rued when I left that old mother of mine.

He is obviously lying.  He is using his mother as a sales pitch to draw the boy in.  Shiflet goes on to say, "My mother was a angel of Gawd," which is ironically what the last Hitch-hiker said about Lucynell (who really was a Child of God, or an Innocent, like the Virgin Mary).  Shiflet says, "He took her from heaven and giver to me and I left her."

And then:

The boy turned angrily in the seat. "You go to the devil!" he cried. "My old woman is a flea bag and yours is a stinking pole cat!" and with that he flung the door open and jumped out with his suitcase into the ditch.

Shiflet had the chance to save Lucynell, an Innocent, but he abandons her in the diner.  He trades her in for the car that Lucynell's mother let him fix up.  The car is a symbol of his traveling "shiftless" faith in God.  It is materialism, his anti-church.  He had the chance to be redeemed, but he refused to save his own life, or the lives of Lucynell and her mother (hence the title, "The Life You Save May Be Your Own").

After abandoning Lucynell, Shiflet feels empty and lonely, so he wants to "save" someone else (who obviously doesn't need saving).  Shiftlet's speech about his mother is very Freudian and full of Christian metaphor (it is about Mother Mary).  The boy listens briefly and then castigates Shiflet for his hypocritical ways and false beliefs.  The boy knows that Shiflet is spewing the Mother rhetoric the way many false preachers spew the Christian rhetoric of faith--as an empty promise, as a means to draw people in and then abandon them.  The boy, unlike Lucynell's mother, has the vision to see through the empty rhetoric and the guts to call out Shiflet for being a false prophet.

Ironically, Shiflet is left alone the same way that he left Lucynell alone.