While attending the University of Giessen, he was inspired by his professor, Baron von Gall, to study the Albigensian (Catharism) movement and the massacre that occurred at Montségur.

Aided by the French mystic and historian Antonin Gadal, Rahn argued that there was a direct link between Wolfram von Eschenbach's Parzival and the Cathars.

In 1934, Rahn published his first book, Kreuzzug gegen den Gral ("Crusade Against the Grail"), which attempted to link the medieval romance of Parzival with the persecution of the Cathars by Pope Innocent III.

Wiligut was impressed by the work and passed it to Heinrich Himmler, the head of the SS, who, from an early age, had a marked interested in ancient history, chivalry and the occult, especially ariosophy.

[2] Rahn joined Himmler's staff as a junior, non-commissioned officer and became a full member of the SS in March 1936, attaining the rank of SS-Unterscharführer the following month.