The two most important types of salvation in Robinson Crusoe are personal (religious) salvation, and physical (safety) salvation. Robinson undergoes both during the course of the novel.
His religious salvation comes after an illness which almost kills him, after which he realizes the importance and personal intervention of God in his many misfortunes:
...as soon as I saw but a prospect of living and that I should not starve and perish for hunger, all the sense of my affliction wore off; and I began to be very easy, applied myself to the works proper for my preservation and supply, and was far enough from being afflicted at my condition, as a judgment from heaven, or as the hand of God against me...
(Defoe, Robinson Crusoe, eNotes eText)
His eventual acceptance of the role of God in his life spurs him to an almost evangelical fervor, and he preaches his beliefs to all who listen.
Crusoe's physical salvation, amusingly, is repeated constantly throughout the first chapters of the novel, as he is shipwrecked multiples times and even sold into slavery, but somehow survives. This continued survival is part of what spurs him to repent and convert, as he realizes that he is surviving through no skill of his own, but "miraculously," through what can only be divine intervention. As he overcomes each physical obstacle, the thought of prayer occurs to him, and is then forgotten in the next crisis. It takes his illness and delirium to hammer the message home. At this point, he stops taking his survival for granted, and works hard in the knowledge that "God helps those who help themselves."