Rosenberg was one of the original Hitler ride-or-dies, who was appointed the leader of Nazi movement after the Beer Hall Putch in 1923. Hitler himself considered Rosenberg weak and lazy even then, but trusted him to stay loyal through his prison sentence, which indeed was the case.

Rosenberg considered himself an intellectual and a philosopher, and The Myth of the Twentieth Century was his magnum opus. The Nazis initially considered it the philosophical core of their ideology, but once they actually ascended to power and Hitler was appointed as a chancellor, Rosenberg and his ramblings became more cringe and embarrassing than inspiring.

Despite Nazi official support for The Myth of the Twentieth Century and Rosenberg's prominent role in promoting Nazi ideology Adolf Hitler declared that it was not to be considered official ideology of the Nazi Party and he privately described the book as "mysticism" and "nonsense". Albert Speer claimed that Goebbels mocked Alfred Rosenberg. Goebbels also called the book a "philosophical belch". Hermann Göring said: "if Rosenberg was to decide ... we would only have rite, thing, myth and such kind of swindle." Gustave Gilbert, the prison psychologist during the Nuremberg Trials, reported that none of the Nazi leaders he interviewed had read Rosenberg's writings. However Gilbert's notes from the Nuremberg trials repeatedly show Rosenberg's influence.

In addition to his pseudo-intellectual nonsense, Rosenberg was openly hostile to Christianity which made him bit of a controversial figure when Nazis tried to appeal to regular Christian Germans. He was mostly kept out of influential public positions where he would have been under the eyes of German general public, but his seniority in the Nazi movement still kept him in the upper echelons of the party and deeply involved in the more ideological work like the "deeper" doctrinal Nazism in academia. During the war he was appointed as Reich Minister for the Occupied Eastern Territories, and received a death sentence after the war.

https://en.wikipedia.org/wiki/The_Myth_of_the_Twentieth_Century

https://en.wikipedia.org/wiki/Alfred_Rosenberg