Shiftlet wants to save his own life, but he can only see this salvation in secular terms. For instance, he abandons Lucynell to, he thinks, save himself. He surrounds himself with false piety and false religious talk that hides from him the reality of his own lack of faith--and the reality that he is not saving his life in any real way.
When he picks up the boy on the side of the road and then converses with him in false pieties about the goodness and purity of their mothers, the boy responds with revulsion and tells the truth. Then he jumps out of the car, saving his own life. He knows Shiftlet is poison.
Shiflet, however, is lost. He projects what he understands as the darkness of the cloud, which is his own darkness, onto the boy, seeing it as "the exact color of the boy's hat." Instead of acknowledging his own inner darkness, he says:
"Oh Lord!" he prayed. "Break forth and wash the slime from this earth!"
Both the boy and the storm are trying to communicate with him to point out his darkness and need of salvation, but he can not "hear" them. The life he saves could be his own, but at this point, it isn't.