Umberto Eco wrote the novel Foucault's Pendulum (1989) that lampooned and satirized motifs found in books like The Holy Blood and the Holy Grail (1982) – Eco also rubbished the “mystery” of Rennes-le-Château in The Book of Legendary Lands, (chapter 14, 2013).

When asked by Elizabeth Vargas about the theory of Jesus Christ and Mary Magdalene, Umberto Eco responded by saying it had no more reality to it than a fairy tale: “The same than the reality of Pinocchio and Little Red Riding Hood” (Nightline, ABC News, 9 May 2008).

Superficial summary of Foucault's Pendulum

The world of secret societies, of masters and initiates, and its exploitation by publishers anxious to feed the public appetite for a cosmic explanation of historical change, has been the subject of a massive satire by the Italian philosopher and philologist, Umberto Eco. In Foucault's Pendulum (1989), three publishers' editors, Belbo, Diotallevi, and Casaubon have been set the task of creating a series of books on the occult by their unscrupulous boss, Signor Garamond. The fashion for Marxist ideology has waned and Garamond spots a gap in the market, with one set of beliefs supplanting another. “It's a gold mine, all right. I realised that these people will gobble up anything that's hermetic, as you put it, anything that says the opposite of what they read in their books at school. I see this also as a cultural duty: I'm no philanthropist, but in these dark times to offer someone a faith, a glimpse into the beyond...” Garamond sees a huge audience, drawn from both the cultists and the academics: “To work, gentlemen. There are libraries to visit, bibliographies to compile, catalogs to request.”

As they assemble material, the three begin to construct their own imaginary version of the secret history of the world, centred upon “The Plan”, as they call it. Encouraged by Casaubon, who has written a thesis on The Templars, they refine the idea of the transmission of The Templars' secret, through many generations and countries, down to the present day. Philip the Fair had naturally wished to gain access to it, for this key was the secret to power unknown in human history. “You aim the right current, stir up the bowels of the earth, and make them do in ten seconds what it used to take them billions of years to do, and the whole Ruhr becomes a diamond mine. Eliphas Levi said the knowledge of the universe's tides and currents holds the secret of human omnipotence.”

The three men, however, underestimate the power of the conspiracy theories to seduce and are themselves drawn in deeper. “I believe” says Casaubon, “that you reach the point where there is no longer any difference between developing the habit of pretending to believe and developing the habit of believing.” They underestimate, too, the determination of these who are really seeking the key to the universe through The Templar secret and will do anything to fulfil the deep psychological need which demands that “The Plan” must really exist. “There can be no failure if there really is a Plan. Defeated you may be, but never through any fault of your own. To bow to a cosmic will is no shame. You are not a coward: you are a martyr.”

Belbo was less easily impressed. When asked, at the end of a late nightdrinking session, how he recognised a lunatic, he was in no doubt: “for him, everything proves everything else. The lunatic is all idée fixe, and whatever he comes across confirms his lunacy. You can tell him by the liberties he takes with common sense, by his flashes of inspiration, and by the fact that sooner or later he brings up The Templars.”