I think it's reasonable to infer that he is a religious man based on his disdain for Wickham, which seems to be based mostly on Wickham's immorality. It's not a given. You could make a case for an atheist Darcy, as an educated man of reason, but I doubt that was Austen's intent. We hear far more about God than science in her books. She wrote a couple of ridiculous clergymen, but they're never her villains.
Besides, if Darcy is not a religious man, then the end of his letter to Elizabeth – the closing that Elizabeth found so moving – becomes extremely cynical.
Besides, if Darcy is not a religious man, then the end of his letter to Elizabeth – the closing that Elizabeth found so moving – becomes extremely cynical.