The Knights Templar
One theory, recently offered up as historical fact in the novel The Da Vinci Code, holds that it came about not as the result of a convergence, but a catastrophe, a single historical event that happened nearly 700 years ago.
The catastrophe was the decimation of the Knights Templar, the legendary order of "warrior monks" formed during the Christian Crusades to combat Islam. Renowned as a fighting force for 200 years, by the 1300s the order had grown so pervasive and powerful it was perceived as a political threat by kings and popes alike and brought down by a church-state conspiracy, as recounted by Katharine Kurtz in Tales of the Knights Templar (Warner Books, 1995):
On October 13, 1307, a day so infamous that Friday the 13th would become a synonym for ill fortune, officers of King Philip IV of France carried out mass arrests in a well-coordinated dawn raid that left several thousand Templars — knights, sergeants, priests, and serving brethren — in chains, charged with heresy, blasphemy, various obscenities, and homosexual practices.
None of these charges was ever proven, even in France — and the Order was found innocent elsewhere — but in the seven years following the arrests, hundreds of Templars suffered excruciating tortures intended to force "confessions," and more than a hundred died under torture or were executed by burning at the stake.
Templar image via Google Images