Short answers, pulled from the story.
Western esotericism is a vast third pillar of Western culture standing alongside doctrinal faith and rationality. The substantive noun esotericism did not appear in English until 1883.
The academic study of Western esotericism was pioneered in the early 20th century by historians of the ancient world and the European Renaissance. Frances Yates and Antoine Faivre are key figures who recognized the structural similarities and crucial role of these movements in Western thought.
The three primary schools of thought are Hermeticism, Gnosticism, and Neoplatonism. Hermeticism produced texts like the Corpus Hermeticum in the 2nd and 3rd centuries, Gnosticism posited that divine light was imprisoned by the Demiurge, and Neoplatonism was advocated by figures such as Plotinus and Porphyry of Tyre.
The Nazi regime prohibited the Fraternitas Saturni in 1936 and arrested its leader Eugen Grosche for a year. While many Nazi leaders were hostile to occultism, Heinrich Himmler used Karl Maria Wiligut as a clairvoyant until Wiligut was forcibly retired from the SS in 1939.
By the 1980s, millenarian currents had come to be widely known as the New Age movement. It became increasingly commercialised as business entrepreneurs exploited a growth in the spiritual market.