Karl von Eckhartshausen was an 18th-century German mystic. The Cloud upon the Sanctuary is Christian mysticism veiled in hermetic code and often considered a classic among Rosicrucians and Theosophists. Eckhartshausen was briefly a member of Adam Weishaupt's Bavarian Illuminati but left for spiritual reasons. In this work, he cryptically mentions a "society of the Elect" that has existed from the very beginning of time as "the invisible celestial Church". He predicted that "it is the society whose members form a theocratic republic, which one day will be the Regent Mother of the whole World." This book would later become a strong influence on the 19th-century members of the Hermetic Order of the Golden Dawn. Arthur Edward Waite offers an informative, historical introduction.