How significant was the stab in the back myth on nazi ideology?

I think it was very significant. Nazi ideology was predicated on the notion of the superiority of the Germanic race both ideologically and culturally. This ideology was incompatible with Germany's total defeat and humiliation in 1918. It suited therefore to perpetuate a myth that Germany had not been defeated but rather "stabbed in the back" by bourgeoise politicians and jews. It allowed soldiers to return "unvanquished" and politicians to rant at the unfairness of the Treaty of Versailles. This was another cornerstone of Nazi ideology.