In Pope Benedict XVI: A Biography of Joseph Ratzinger, John L. Allen Jr. writes about the relationship between the Catholic Church and the Nazi Party:
Many ordinary Catholics objected to attacks on their church, but there was simply no opposition to Nazism tout ensemble. ... In fact, there were key points at which Nazi and Catholic attitudes intersected and created a basis for mutual support. Both groups hated the Weimar Republic. The Nazis opposed Weimar because it was allegedly too Jewish and led by the “November Criminals” who sold out the country after the First World War; Catholics objected to it because it smacked of liberalism, sexual degeneracy, and an irreligious spirit.
Cardinal Faulhaber, for example, gave a speech in May 1933 in which he expressed thanks for the Volksgemeinschaft, or spirit of community, which Hitler had fostered, and rejected “liberal individualism.” Moreover, Catholics shared with Nazis an instinctive fear of the Bolsheviks.
Finally, there was a form of anti-Jewish sentiment that was openly accepted among Catholics, based in part on the theological argument that the Jews sinned by rejecting Christ and in part on the historical fact that many Jews had played leading roles in the Kulturkampf. As early as 1925, a Franciscan priest named Erhard Schuland wrote a book called “Katholizismus und Vaterland” (Catholicism and Fatherland) that called on Germans to fight “the destructive influence of the Jews in religion, morality, literature and art, and political and social life.” Schuland expressed what was very much the consensus in German Catholicism of the day...
Support for the Nazis, their social policies, and their anti-Semitism was not limited to ordinary Catholics and a few random priests:
Archbishop Konrad Gröber of Freiburg was known as the “Brown Bishop” because he was such an enthusiastic supporter of the Nazis. In 1933, he became a “sponsoring member” of the SS. After the war, however, he claimed to have been such an opponent of the Nazis that they had planned to crucify him on the door for the Freiburg Cathedral.
Bishop Wilhlem Berning of Osnabrück sat with the Deutsche Christen Reichsbishop in the Prussian State Council from 1933 to 1945, a clear signal of support for the Nazi regime.
Cardinal Bertram also had some affinity for the Nazis. In 1933, for example, he refused to intervene on behalf of Jewish merchants who were the targets of Nazi boycotts, saying that they were a group “which has no very close bond with the church.”
Bishop Buchberger of Regensburg called Nazi racism directed at Jews “justified self-defense” in the face of “overly powerful Jewish capital.”
Bishop Hilfrich of Limburg said that they true Christian religion “made its way not from the Jews but in spite of them.”
Because the Catholic leadership did not consistently oppose the Nazi policies, it was relatively easy for the Nazis to co-opt the Catholic churches in their effort to round up and exterminate the Jews. A large number of Jews converted to Christianity in order to avoid persecution and the only way the Nazis found them out was because of the help of Catholic authorities:
After April 7, 1933, civil servants in Germany were required to prove that they were not Jews. Because births had been registered by the state only since 1874, the church was called upon to provide many records. The Catholic church cooperated right up to the end of the war. Likewise, after the 1935 Nüremberg laws that forbade marriage between Aryans and non-Aryans, most Catholic priests did not perform such ceremonies, even though the number of Jewish conversions to Catholicism was accelerating because of the persecution.
Yes, right up until the end of the war, Catholic clergy were actively assisting the Nazi program of racial purification. They provided detailed records of who converted and who didn't, who married and Jew and who didn't. When two people wanted to marry, Catholic priests enforced Nazi race laws against Aryans being allowed to marry non-Aryans. The Nazis' agenda of racial discrimination and purification would not have worked without the active, willing, and eager cooperation of Christian churches.
After the war, the Allies tried to rely on Catholic clergy to help them in their program of de-Nazification of the government. That was a mistake — Catholic assistance to the Nazis hadn't ended when the Nazis surrendered. Catholic bishops realized that eliminating all Nazis would leave Communists and Social Democrats in charge and they concluded that that would be worse than having the Nazis in power — so they basically lied to the Allies. Unrepentant Nazis were returned to positions of authority over the German people because Catholic clergy gave them a clean bill of political and ideological health.
Eventually the Allies grew wise to the Catholic duplicity and stopped relying on the word of priests about whether someone had been a Nazi. That is the legacy of the Catholic Church from Nazi Germany: not resistance, but cooperation; not the defense of principle but the defense of social power.