He begins his time on the island without much hope and in much distress. The major theme of the book, though, is how he wrestles with God. He begins his time on the island wondering why God would have put him there. It's a very hard struggle and he basically rejects all thoughts of God. At one point he says this:
God had appointed all this to befall me...[and] I was brought to this miserable circumstance by His direction, He having the sole power, not of me only, but of everything that happened in the world.
Over time he continues to survive and have all of his basic needs met. Toward the end of the book, after 15 years or so on the island, he is praising God for His sovereignty and supplying for His needs.
God...comfortably provided for me in my desolate condition. I learned here again to observe that it is very rare that the providence of God casts us into any condition of life so low, or any misery so great, but we may see something or other to by thankful for; and may see others in worse circumstances than our own.
Also, throughout all this, he ends up befriending a native of the island, who he names Friday. In reflecting on this situation, he speaks quite well to those who wonder about others in remote parts of the world, who have never heard the gospel. He says:
Why has it pleased God to hide the like saving knowledge from so many millions of souls, who would make better use of it than we did?
From hence I sometimes was led too far to invade the sovereignty of Providence and, as it were, arraign the justice of so arbitrary a disposition of things that should hide that light from some and reveal it to others, and yet expect a like duty from both. But I shut it up an checked my thoughts with this conclusion, first, that we did not know by what light and law these should be condemned; but that, as God was necessarily, and by the nature of His being, infinitely holy and just, so it could not be but that if these creatures were all sentenced to absence from Himself, it was on account of sinning against that light which, as the Scripture says, was a law to themselves, and by such rules as their consciences would acknowledge to be just, thought the foundation was not discovered to us. And secondly, that still, as we are all the clay in the hand of the Potter, no vessel could say to Him, 'Why hast Thou formed me thus?
In conclusion, I'm challenged by his reverence for God in the hard circumstances as well as his determination to let the Word speak truth to him, despite what his emotions might lead him to believe. It's a good one, I recommend it.