Pope Pius XII (pictured) published his first encyclical, Summi Pontificatus, which critiqued ideologies such as racism, cultural superiority and totalitarianism.

Pope Pius XII was the head of the Catholic Church and sovereign of the Vatican City State from 2 March 1939 until his death on 9 October 1958. He is the most recent pope to take the pontifical name "Pius".

An encyclical was originally a circular letter sent to all the churches of a particular area in the ancient Roman Church. At that time, the word could be used for a letter sent out by any bishop. The word comes from the Late Latin encyclios. The term is now primarily associated with papal encyclicals.

Summi Pontificatus is an encyclical of Pope Pius XII published on 20 October 1939. The encyclical is subtitled "on the unity of human society". It was the first encyclical of Pius XII and was seen as setting a tone for his papacy. It criticizes what it saw as major errors of the time, such as ideologies of racism, cultural superiority, and the totalitarian state. It also sets the theological framework for future encyclical letters such as Mystici corporis Christi (1943). The encyclical laments the occupation of Poland, denounces the Molotov–Ribbentrop Pact, and calls for a restoration of independent Poland.