I believe it was Kirkegaard that divided the world into two basic realms that of "the rational" and that of "the non-rational."

He basically said that the rational area is useful in defining things but strips everything of value that the idea of value comes from the non-rational which is given via a "leap of faith." (NOTE: this did not have the same meaning as it does today) So we have 2 worlds that NEVER intersect the rational and the non-rational. Things can be described by the first or the second but the two cannot be combined.

I think he would have laughed his ass off at the way his ideas are being used today. Belief in ANYTHING is seen as non-rational in that form of system you can only describe how things are if certain premises are true but you can never validate the premises. In that system all premises are equally valid.

"Unicorns are our secret overlords from outerspace" has the same amount of "objective proof" (ie none from a rationalistic view) as does the statement "The sun rotates about the earth roughly once every 365.25 days."

So in his view you can believe whatever you want because all premises have equal validity and only the non-rational can provide "meaning" for your life (which you will provide yourself or end up commiting suicide).

Some people took his ideas and focused on the supremacy of the rational which led to nihilism, logical positivism, etc. Some focused on the non-rational aspect and developed the existenialist philosophies, mysticism, and relativistic morality ideas.