This belief contains a few grains of actual reality. In 1776, Bavarian university professor Adam Weishaupt founded a club called the Order of Perfectibilists, aka the Illuminati. Basically, they were a bunch of guys – no more than a few hundred, at the movement's peak – who copied some of the mystical symbolism from the more popular Masonic lodges. They sat around kvetching about the German nobility and the Jesuit religious order, and waxing philosophical about egalitarianism. They were fairly harmless, but even so, in 1785, Bavarian Elector Prince Karl-Theodor – apparently a bit thin-skinned – ordered the Illuminati and other secret associations to disband [source: McConnachie and Tudge].
But the Illuminati name was too cool to fade away. When arrested in Rome in 1789, an occultist and faith healer named Cagliostro appeased the Inquisition by confessing to knowledge of a secret group that, among other things, was plotting to overthrow the papacy [source: McConnachie and Tudge].
The story was so fascinating that it mutated and jumped the Atlantic. In 1798 in Massachusetts, the Rev. Jedidiah Morse denounced the Illuminati from the pulpit, and when pressed, fingered conspicuously secular Thomas Jefferson as its U.S. leader. A pamphlet that accused the imaginary group of being "indefatigably engaged in destroying the religion and government of the United States" soon followed [source: Knight]. In the years after, the Illuminati became conspiratorial fodder for everyone from the John Birch Society to author Dan Brown, whose 2000 novel "Angels and Demons" uses them as villains.