75 years ago today, on March 21, 1937, the papal encyclical Mit brennender Sorge (with burning concern) was read in all Catholic churches of Germany.
It had been prepared and copied in secret, for doing that openly was impossible, Germany already being a totalitarian state at that time. The content was an explanation of the incompatibility between National Socialist ideology and Christianity. The Vatican has both the German original and an English translation online. Money quotes:
Whoever exalts race, or the people, or the State, or a particular form of State, or the depositories of power, or any other fundamental value of the human community - however necessary and honorable be their function in worldly things - whoever raises these notions above their standard value and divinizes them to an idolatrous level, distorts and perverts an order of the world planned and created by God; he is far from the true faith in God and from the concept of life which that faith upholds.
The peak of the revelation as reached in the Gospel of Christ is final and permanent. It knows no retouches by human hand; it admits no substitutes or arbitrary alternatives such as certain leaders pretend to draw from the so-called myth of race and blood. Since Christ, the Lord's Anointed, finished the task of Redemption, and by breaking up the reign of sin deserved for us the grace of being the children God, since that day no other name under heaven has been given to men, whereby we must be saved (Acts iv. 12). No man, were every science, power and worldly strength incarnated in him, can lay any other foundation but that which is laid: which is Christ Jesus (1 Cor. iii 11). Should any man dare, in sacrilegious disregard of the essential differences between God and His creature, between the God-man and the children of man, to place a mortal, were he the greatest of all times, by the side of, or over, or against, Christ, he would deserve to be called prophet of nothingness, to whom the terrifying words of Scripture would be applicable: "He that dwelleth in heaven shall laugh at them" (Psalms ii. 3).
The Nazi government reacted like totalitarian governments typically do: It ensured the press didn't mention the encyclical. Then it closed Catholic institutions of education, seized the printing shops that had participated in copying the encyclical and staged show-trials against Catholic clergy for sexual immorality.
But most of this was just an acceleration of what they already had been doing. Basically they kept it quiet and intensified their program of marginalizing the Church. And it worked. Annexing Austria and the Sudetenland a year later, Hitler rose to new hights of popularity. Everyone knows what happened after that. Any internal opposition remained limited to historical footnotes. Germany no longer was the kind of country where speaking out has political effects.