"None"
There you go. Couldn't quite come up with such a reasonable answer in the emergency room at 2 AM.
You'd think it was reasonable, but there are no guarantees.
I went to the emergency room myself once, having cut my thumb open on a soup can lid. The admitting nurse went thru the usual checklist, then when she got to "Religion?", I simply said, "None." She paused, then looked up with a frown and squinted at me as though she were trying to look thru a dirty window.
"No religion...?"
I nodded. "Yes, that's right," and didn't offer anything further, my rationale being that I was there for medical treatment, not anything else. I was wondering whether she might pursue the matter, because I could tell she definitely wanted to. Instead, though, she stared at me for a moment, then shrugged and went back to the checklist. It was very unprofessional of her, of course, but then, lots of people are unprofessional -- what else is new?