One of the ways that Defoe addresses salvation through Robinson Crusoe is in the story being a metaphor of sorts for the way salvation is presented in the Christian theology. When Robinson decides that he cannot approach life the way he is expected to within society, he leaves and winds up on the deserted island trying to figure out how to live in a very different sense. This rejection of the status quo or of the world's expectations serves as his original sin, that which separates him from God, or in this case his fellow human beings and the society into which he was born.
The struggle for redemption is what drives the plot in the book as Robinson begins to find meaning in his life in a very different way than he had prior to his shipwreck.
This mimics some of the ideas about salvation and dissention that were present in society at the time. There was a strong culture of obedience and Defoe made his statements about that culture through the character of Robinson Crusoe. Just as some people left England to escape the strictures of puritanism and the dominant culture of the time, Crusoe escapes to a deserted island in order to find his own way in life.