>Christian references in the Cantos were probably incidental,

They're not at all incidental. The themes and the literal Catholic Church don't just make it into the books by osmosis, they're central to it and deliberate.

Like Gene Wolfe he's part of a pretty small group of US authors who wrote Catholic speculative fiction. Like Wolfe his writing is also fairly un-American. If Heinlein or Asimov are examples of archetypal US science fiction, Simmons is about as far as the other end as you can be, with the post-modern structure, the Canterbury Tales as a template for the story and so on.