My quote is from Michael Aquino, founder of the Temple of Set, in his reply to an anti-Satanist diatribe by Isaac Bonewits, a neo-Druid, back in 1991. Aquino wrote:
Satanism is actually based on two very simple principles: first, that the individual human consciousness is a free agent apart from the non-conscious forces of nature; and second, that this fact is so frightening to most people that they have "demonized" it and either suppressed, punished, or sublimated its influence on themselves.
I should emphasize here that Aquino was speaking only for the Temple of Set. As far as I can tell, most Satanists would not agree with the above as a definition of Satanism.

In any case, I don't think it's true that the idea "that the individual human consciousness is a free agent apart from the non-conscious forces of nature" has been particularly "demonized," either by Christianity or by our culture in general. Christian churches have demonized lots of things, but the above idea doesn't seem to be one of them.