Partly Responsible? No more "responsible" than dozens of other social, cultural, economic and political factors
While Forsythe's assertion as written is piece of artistic license, there was an association of pre-Christian symbolism, as reflected in such cultural icons as Richard Wagner's operas featuring pre-Christian German legends such as Der_Ring_des_Nibelungen.
Cultural nationalism had been a factor in the previous century's establishment of the German Reich (Bismarck and the Kaisers). Hitler wasn't doing anything new, he was building on what Bismarck had managed about a half of a century before Hitler tried his hand at establishing a long lasting Reich. Cultural nationalism was one of his many themes used for political purposes.
... in the early phase, cultural nationalists worked to recover or re-invent traditional cultural stories in order to argue for the importance of their people’s cultural experience. Then they used this unified canon (an established past) as a point to rally their people (an activist present) to forge a united nation (a proposed future). We see this pattern in Wagner as well.
Hitler and his party appealed to (among other things) nationalism in their efforts to take Germany in their political direction. FWIW, Wagner's legacy has in the last half of a century certainly come under fire, in part due to an association with nationalism and anti Semitism, which is also associated with Hitler and the Third Reich.
Your question suggests that one should consider the social context when that novel was published.
I read The Odessa File shortly after it came out. (Early 1970's.) At that time, the hunt of for some of the Nazis who had escaped to Argentina, Brazil, and elsewhere was alive in the Western context. It was also in the news now and again as various SS or other sorts were found.
This was an era when WW II movies were still very popular (The Longest Day, Raid on Rommel, the Dirty Dozen, the Boys from Brazil were all contemporary films to the novel's release). The Nazi was a standard trope-villain in both film and literature. (Robert Ludlum featured a few Nazis as the bad guys, or shadow bad guys, in his books, also popular at the time).
Given the time of Fortsythe's book's publication, his alluding to that symbolism -- the pre-Christian Germanic Volk and their legends/paganism -- is not a surprise. It was par for the course in that time and place for fictional treatments of the Third Reich. Given the sales of the book, and the movie after, it was part of a successful formula used by Forsythe and others.