From my Q&A with Linda Bridges:
LOPEZ: This is reflected in the book a bit: Bill was remarkably comfortable with piety, for such an intellectual, wasn’t he? What accounts for that?
BRIDGES: I suspect because it was so much a part of him, so much a part of the world that shaped him. His parents were both devout, and besides loving them deeply he had a very high regard for them. So it always seemed natural to him that very bright people should be religious. He later, of course, came to meet many who were not, but that never shook his own faith. He didn’t proselytize, but if a reference to God or to Christian teaching was relevant to his argument, I suspect it never occurred to him not to use it.