Wikipedia
The Reichskonkordat (" Concordat between the Holy See and the German Reich") is a treaty negotiated between the Vatican and the emergent Nazi Germany. It was signed on 20 July 1933 by Cardinal Secretary of State (and later Pope Pius XII) Eugenio Pacelli on behalf of Pope Pius XI and Vice Chancellor Franz von Papen on behalf of President Paul von Hindenburg and the German government. It was ratified September 10, 1933 and it has been in force from that date right up until the current day. The treaty guarantees the rights of the Roman Catholic Church in Germany. When bishops take office Article 16 states they're required to take an oath of loyalty to the Governor or President of the German Reich established according to the constitution. The treaty also requires all clergy to abstain from working in and for political parties. Nazi breaches of the agreement began almost as soon as it had been signed and intensified afterwards leading to protest from the Church including in the 1937 Mit brennender Sorge encyclical of Pope Pius XI. The Nazis planned to eliminate the Church's influence by restricting its organizations to purely religious activities.
The Reichskonkordat is the most controversial of several concordats that the Vatican negotiated during the pontificate of Pius XI. It is frequently discussed in works that deal with the rise of Hitler in the early 1930s and the Holocaust. The concordat has been described by some as giving moral legitimacy to the Nazi regime soon after Hitler had acquired quasi- dictatorial powers through the Enabling Act of 1933, though Reichskanzler Hitler himself is not a signatory to the treaty and the treaty does not make mention of Hitler, or the Nazi Party.
The treaty places constraints on the political activity of German clergy of the Catholic Church. This contributed to a decrease in the previously vocal criticism of Nazism by the hierarchy of the Catholic Church in Germany, after September 1933 when the treaty was ratified. From a Roman Catholic church perspective it has been argued that the concordat prevented even greater evils being unleashed against the Church, although critics like Gregory Paul call it a "classic political kickback scheme". Though some German bishops were unenthusiastic, and the Allies at the end of World War II felt it inappropriate, Pope Pius XII successfully argued to keep the concordat in force. It is still in force today.