The term used first of all in the 15th century by enthusiasts in the occult arts signifying those who claimed to possess "light" directly communicated from a higher source, or due to a larger measure of human wisdom. We first find the name in Spain about the end of the 15th century. Its origin is probably a late Gnostic one hailing from Italy, and we find all sorts of people, many of them charlatans, claiming to belong to the brotherhood. In Spain, such persons as laid claim to the title had to face the rigour of the Inquisition, and this is perhaps the reason that we find numbers of them in France in the early seventeenth century, as refugees.
Here and there small bodies of those called Illuminati, sometimes known as Rosicrucians rose into publicity for a short period. But it is with Weishaupt, Professor of Law at Ingolstadt, that the movement first became identified with republicanism. It soon secured a strong hold all through Germany, but its founder's object was merely to convert his followers into blind instruments of his supreme will. He modelled his organisation on that of the Jesuits, adopted their system of espionage, and their maxim that the end justified the means. He induced mysficism into the workings of the brotherhood, so that an air of mystery might prevade all its doings, adopted many of the classes and grades of Freemasonry, and held cut hopes of the communication of deep occult secrets in the higher ranks.
Only a few of the members knew him personally, and thus although the society had many branches in all parts of Germany, to these people alone was he visible, and he began to be regarded by those who had not seen him almost as a god. He took care to enlist in his ranks as many young men of wealth and position as possible, and within four or five years the power of Illuminism became extraordinary in its proportions, its members even bad a hand In the affairs of the state, and not a few of the German princes found it to their interest to having dealings with the fraternity. Weishaupt's idea was to blend philanthropy and mysticism. He was only 28 when he founded the sect in 1776, but he did not make much progress until a certain baron Von Knigge joined him in 1780. A gifted person of strong imagination he had been admitted master of most of the secret societies of his day, among them Freemasonry. He was also an expert occultist and the supernatural had strong attractions for him. These two, rapidly spread the gospel of the Revolution throughout Germany. But they grew fearful that, if the authorities discovered the existence of such a society as theirs they would take steps to suppress it. With this in view they conceived the idea of grafting it on to Freemasonry, which they considered would protect it, and offer it means of spreading more widely and rapidly.
The Freemasons were not long in discovering the true nature of those who had just joined their organisation. A chief council was held with the view of thoroughly examining into the nature of the beliefs held by them and a conference of masons was held in 1782 at which Knigge and Weishaupt attended and endeavoured to capture the whole organisation of Freemasonry, but a misunderstanding grew up between the leaders of illuminism. Knigge withdrew from the society, - and two years later those who bad reached its highest grade and had discovered that mysticism was not its true object, denounced it to the Bavarian Government as a political society of a dangerous character. Weishaupt fled, but the damage had been done, for the fire kindled by Illuminism was soon to burst forth in the French Revolution.
It has been suggested, and must certainly be true that the fathers of the United States had a solid grounding in the same principles as Weishaupt, and that the declarations of 1776 were not entirely coincidental to his founding of the German movement. Certainly it can be reasonably suggested that the American Freemason lodges provided Weishaupt with an example, and there is fair reason to belive that Washington, Jefferson, and other Masonic luminaries of the United States Independence corresponded with the German society of the same aim.
The title Illuminati was later given to the French Martinists (q.v. )