As a mariner and traveler, the sea plays a pretty big part in Crusoe's life. Whenever a storm hits the ocean, Crusoe is immediately penitent and begs God for help. When the skies are clear and the waves are calm, Crusoe seems to forget all about that religious stuff. For example:
In a word, as the Sea was returned to its Smoothness of Surface and settled Calmness by the Abatement of that Storm, so the Hurry of my Thoughts being over, my Fears and Apprehensions of being swallow'd up by the Sea being forgotten, and the Current of my former Desires return'd, I entirely forgot the Vows and Promises that I made in my Distress. (10)
The ever-changing sea serves as a useful metaphor for Crusoe's fickle relationship with God.
Three Bibles are among the wreckage of the ship. The Bible is hugely important for Crusoe's time on the island, as it will serve as his moral compass and means of spiritual reformation.
…also I found three very good Bibles which came to me in my Cargo from England, and which I had pack'd up among my things. (56)
The book is a symbol of Crusoe's connection to God and later becomes a tool with which to teach Friday the basics of Christianity.
Crusoe might have been spiritually sick, but it's not until he becomes physically ill that he decides to save his soul. In the grips of the flu (or "Ague" as Crusoe calls in on page 75), Crusoe has a fever dream in which a man comes down from the heavens and admonishes him for not yet repenting, and tries to kill him with a spear. He begins to see that his past behaviors have been sinful and his present miseries are punishments for his rebellious behavior. He finally repents and utters his first prayer. This is the beginning of Crusoe's spiritual life.
Crusoe sees that fatal on the island and realizes that (gasp!) he's not alone. He describes the scene as follows:
It happen'd one Day about Noon going towards my Boat, I was exceedingly surpriz'd with the Print of a Man's naked Foot on the Shore, which was very plain to be seen in the Sand: I stood like one Thunder-struck, or as if I had seen an Apparition; I listen'd, I look'd round me, I could hear nothing, nor see any Thing, I went up to a rising Ground to look farther, I went up the Shore and down the Shore, but it was all one, I could see no other Impression but that one, I went to it again to see if there were any more, and to observe if it might not be my Fancy; but there was no Room for that, for there was exactly the very Print of a Foot, Toes, Heel, and every Part of a Foot; how it came thither, I knew not, nor could in the least imagine. But after innumerable fluttering Thoughts, like a Man perfectly confus'd and out of my self, I came Home to my Fortification, not feeling, as we say, the Ground I went on, but terrify'd to the last Degree, looking behind me at every two or three Steps, mistaking every Bush and Tree, and fancying every Stump at a Distance to be a Man; (130)
Why might the footprint be terrifying to Crusoe?
The cannibals on the island offer Crusoe an opportunity to reflect on the differences between cultures. Should he intervene in their affairs or not? Should he judge their actions, or leave that up to God? He reflects on the topic in the following passage:
Religion joyn'd in with this Prudential, and I was convinc'd now many Ways, that I was perfectly out of my Duty, when I was laying all my bloody Schemes for the Destruction of innocent Creatures, I mean innocent as to me: As to the Crimes they were guilty of towards one another, I had nothing to do with them; they were National, and I ought to leave them to the Justice of God, who is the Governour of Nations, and knows how by National Punishments to make a just Retribution for National Offences; and to bring publick Judgments upon those who offend in a publick Manner, by such Ways as best pleases him. (146)
After much thought, Crusoe realizes that he cannot kill the cannibals, as that would be taking up God's office. It's up to God to punish nations of people who do wrong, not the individual man. Note, however, that Crusoe does later decide to intervene in the cannibals' actions when he sees that they are ready to kill and eat a Spaniard. Why?