Your attention is directed to the Barmen Declaration.
An admirable work, and good so far as it goes. I have no doubt that there were many German non-Catholics of good will, but the encyclical is more detailed, more confrontational, and read via a covert operation by suprise in every single Catholic Church in Germany on passion Sunday—which was a big deal under the old calendar. Moreover, its public reading followed by a few days the public issuing of an encyclical against communism, which had given the Nazis a few days to say “listen to the Pope agreeing with us” before being blind sided and made to look like idiots.
I have no doubt that voluntary protestant unions could be eloquent and had individuals making extremely heroic acts, but I do not think they could cause a public humiliation of this magnitude.