I think faith is believing something that you know to be true but can neither prove nor disprove in a way that will satisfy another person.

I like that.

As for finding the context, I kind of took it up as a personal challenge. I couldn't imagine someone like Twain saying it in all seriousness when sarcasm was his weapon of choice. It did, as it turned out, fit his pattern of at least one idiot for each village his imagination built. It is scary what people will draw out of a single line from a massive dialogue.