Why did Adolf Hitler believe that they were stronger than the Jews?

Answer 1

He may not have believed that they were stronger, but he certainly thought that they were better.

In terms of the motivations behind Hitler believing that the Aryan race was inherently superior, there really were no arguments or evidence that withstand any serious scrutiny. He based some of it on pseudo-science (which involved skull measurements done with shoddy accuracy); he based some of it on cultural pronouncements (which were loaded with natural Anti-Semitic bias); he based some of it on Anti-Semitic forgeries (like the Protocols of Zion which has long been known to be a hoax) and numerous other issues. The justifications for German superiority are just as ridiculous as the claims of White superiority in the United States during the 1700s and 1800s as concerns Blacks.