Fideism is faithism, a reliance upon faith - absent of reason or logic. It's an epistemological theory which maintains faith is independent of reason. Example: The classic belief that there is a God, based on faith, not on logical reasoning. Religious arguments are more appealing when phrased in fideism because it deals away with skepticism in such a manner that one could be called a heretic for divulging a logical discrepancy.

Besides the powerful French Catholic bishop mentioned by rahmat as Cardinal du Perron, there was another erudite who spent most of his entire life reading. Pierre Baye, a French-speaking Protestant erudite, philosopher, and theologian, was exiled in Holland. Both of our scholars believed that any event can have an infinity of possible causes. What I would dub the anti-thesis to fideism. (There can only be one cause, God planned it.)

You cannot prove or disprove God exists with reason according to fideism. You can choose to believe or disbelieve that there is a God. The Black Swan contends that human reason is debunk. The turkey that is fed by its master for 1,000 days can never use its past experience to actually predict what will happen come that Wednesday before Thanksgiving. It will be slaughtered by the same hand that fed it for nearly 3 years. Human reason does not account for Black Swans, the impact of the highly improbable. It is not a stretch to say that we are all fideists, because we all believe, without certainty, that the next day will bring similar events to our past experience. We cannot predict with certainty when an earthquake will hit and change our lives forever.

However, those labeled fideists do not self proclaim themselves in such a manner. It's actually a derogatory term used by opponents of a fideist. Support of fideism is most commonly ascribed to four philosophers: Pascal, Kierkegaard, William James, and Wittgenstein.

"Plantinga writes, a fideist is someone who 'urges reliance on faith rather than reason, in matters philosophical and religious'." (Standford) You should not use this framework in say economics. But tying that into the Black Swan, that's exactly what Taleb says we do. That's why we're so blind to predict stock market crashes, or unforseen events that dramatically redirect the course of history.

"In contrast to the more rationalistic tradition of natural theology, with its arguments for the existence of God" (Standford) fideism is the only framework that does not allow for the existence of reason, ie standard arguments of Gods existence. Even the Catholic Church contends there are logical reasons that prove Gods existence.

Sources: The Black Swan, by Nassim Nicholas Taleb.