I haven’t been nice to Hobbes for a bit, so:
When God speaketh to man, it must be either immediately or by mediation of another man, to whom He had formerly spoken by Himself immediately. How God speaketh to a man immediately may be understood by those well enough to whom He hath so spoken; but how the same should be understood by another is hard, if not impossible, to know. For if a man pretend to me that God hath spoken to him supernaturally, and immediately, and I make doubt of it, I cannot easily perceive what argument he can produce to oblige me to believe it.
There. Isn’t that wonderful? It so beautifully turns around the “You say God told you that but I think you’re a fraud” into “I really can’t see how you could convince me of that”. There is more, of course. [[Leviathan (book)]] provides an intro, and as it happens I wrote it (or almost all of it) and it has survived remarkably well. The section Of a Christian Common-wealth is good fun: here Hobbes tries to make a case for which books of the Bible you can reliably believe in, but (much like Popper on rationalism) is eventually obliged (oh dear, he really didn’t want to go that way :-) to provide an external authority to decide which books can be trusted: the Civil Power in his case, of course.
Ah, and now of course I’ve remembered what I actually intended to write about: Atmoz’s 400 ppm CO2 challenge! Off you go; I haven’t made my mind up yet.