On 20 July 1933, the Holy See and the German Reich concluded the Reichskonkordat, a treaty on the Catholic Church in Germany. It is still in force. Article 21 makes Catholic religious instruction a regular school subject. On 26 March 1957, the Federal Constitutional Court ruled on the treaty. It found that the concordat had been validly concluded and never terminated. Violations had led to protests, not to its end. The concordat therefore still existed and bound the Federal Republic.
The same ruling let the states depart from the treaty’s school rules under German law, while the treaty still bound Germany under international law.