DeFoe's protagonist becomes a religious man because of his labor and (eventual) deference to God.
George Starr argues in his article "Robinson Crusoe and the Myth of Mammon," that the 17th century faithful believed hard work must be coupled with a dependence on God. "Providence," Starr says, "does not excuse man from action but calls him to it and sustains him in it."
Defoe expresses this sentiment in an essay he titled, "Serious Reflections," saying: "(W)e are to trust Providence with our estates, but to use, at the same time, all diligence in our callings, so we are to trust Providence with our safety, but with our eyes open to all its necessary cautions, warnings, and instructions."
This is precisely what brings Crusoe around: accepting hard work and trusting in God to sustain him. One is useless without the other. It is not, as Star argues, "either/or" but rather "both/and." Eventually, Crusoe understands this and he learns "thankfulness and resignation."