I'd probably distinguish a statement as 'fiction' or 'nonfiction' based on what it's purported to be by the author.

Statements of fiction are those that are purported to be completely invented in people's heads, whereas statements of nonfiction are those purported to have a basis in our shared space (i.e. "reality").

So the key difference here is not whether the statement is "true" or useful, but rather, how the recipient is asked to think about it.

Statements purported to be nonfiction require you to put on your thinking cap, and decide whether they are worth adding to (or rejecting from) your worldview. Whereas statements of fiction ask you to do the opposite - to suspend your disbelief, and not to evaluate them for adoption into your worldview.

By this standard, I would, technically, in fact, consider statements of faith and religion to be statements of nonfiction.

But that doesn't necessarily mean that I believe them to be true, useful, or honest.