In short, as a guys who studies history daily at uni and hopes to make it a career, My opinion is that protestantism had little direct relationship to Nazism’s rise in Germany.
Hitler was raised a Catholic in Austria, and the undertones and philosophical movements all credited with leading to national socialism tend to be products of the 18th and 19th century. The primary one being the rise of nationalism, particularly Ethnic nationalism. As central governments became more powerful during the second half of the last millennium and large, powerful nation states emerged it became important to unify a populace which had previously lived in a feudal, regionalist society. So ethnic decent, religion, and common language became important tools of the state, as did demonizing or belittling other large ethnic groups.
Now, as time goes on people are born and eventualy live their whole lives under the new system, culture adjusts, and by the1930’s country’s like France and Britain had been living with a strong national system for centuries. But Germany on the other hand had only been a single unified nation for 60 years. Just a hundred years before the were over 350 different independent states within German Speaking Europe, I believe the famous quote is “one for every day of the year”.
Now Germany did experience a massive boon in nationalism in the 40 years leading up to WWI. They defeated the French with almost comical ease in 1872, uniting the country in the proses, and quickly rose to become the most powerful nation in the world. Then in the Great war they swept through Belgium into France, Eventually toppled Tsarist Russia, defeated the Royal Navy at Jutland, and had been told for for years the German army was undefeated in the field, when suddenly they lost. It was a shock to the young and fragile system. The people rose up, the military mutinied, Bavaria even attempted to reestablish its independence. Follow that with the absolutely humiliating treaty of Versailles, the forced annexation of German speaking areas by Poland, and then one of the worst depressions ever seen in history and Germany was a once proud nation of shattered confidence. It was the perfect environment for fascist racism to rise to prominence.
Protestantism may have slightly made Germany more vulnerable to Nazism (though Remember Southern Germany and Austria is Catholic) but it seems the nationalist tones of the previous century and the geopolitical disaster that was the previous two decades for Germany had more to contribute than religion. The Jews were different, they looked different, they had their own language in some areas, they were already stereotyped as rich bankers, they were perfect scapegoats.