Discuss Robinson Crusoe as a religious allegory.

Robinson Crusoe is intended to be an allegory for what happens when we turn away from God. Crusoe has been washed up on a desert island where he's been provided with everything he needs to survive. Yet far from offering thanks to God for such bounties, he takes everything for granted. Unlike most Christians at the time, he doesn't see the benevolent hand of God at work: he doesn't see Providence.
It was such arrogance that originally led Crusoe to ignore his father's wishes and take to the high seas. In defying his father, Crusoe was also defying the Almighty, challenging the God-given social order in which fathers rule over their children as divine surrogates. It's not too hard to see this as an allegory on humankind's sinful tendency to turn its back on God the Father and try to make its own way in the world, without divine grace. This would've been an especially important theme for Puritans like Daniel Defoe, who believed that a sinful humanity's salvation was totally reliant on the freely given gift of God's grace.
It's only when Crusoe undergoes a profound religious conversion that he returns to the path of righteousness. But even then there are many bumps along the way, emphasizing once more the inherently sinful nature of man.

During his time on the island, Robinson Crusoe goes through a religious conversion. He was raised Puritan, but he commits sins such as rebelling against his father and failing to appreciate his class status. These decisions and feelings go against God. He suffers trials and misfortunes, ultimately winding up on an abandoned island, but his conversion is not complete until after he falls sick and nearly dies as a result of fever. He calls on God to deliver him and vows to serve Him afterward.
This story contains some parallels with the prodigal son parable in the Bible, in which the son leaves his father and squanders his fortune. When he returns, the father welcomes him with open arms.
Robinson Crusoe goes on to record reflections about his relationship to God, as well as his place in creation (which he had been dissatisfied with before). It is important to note that he also teaches Friday religion.