Humani generis unitas (Latin; English translation: On the Unity of the Human Race) was a draft for an encyclical planned by Pope Pius XI before his death on February 10, 1939. The draft text condemned antisemitism, racism and the persecution of Jews. Because it was never issued, it is sometimes referred to as "The Hidden Encyclical" or "The Lost Encyclical". Humani generis unitas was written by three Jesuits under the leadership of John LaFarge. The draft text remained secret until published in 1995 in France (by Passelecq and Suchecky under the title L'Encyclique Cachée de Pie XI) and in 1997 in English as The Hidden Encyclical of Pius XI.
In June 1938, Pius XI assigned American Jesuit John LaFarge to prepare a draft of Humani generis unitas. Jesuit Superior-General Wlodimir Ledóchowski assigned two other Jesuits, Gustav Gundlach and Gustave Desbuquois, to assist LaFarge. Working in Paris, they produced a draft of approximately 100 pages. Another Jesuit translated the draft into Latin and presented it to Ledóchowski. The draft was delivered to the Vatican in September 1938.
Pius XI's successor, Pope Pius XII, did not promulgate the draft as an encyclical. Critics of Pius XII (notably John Cornwell in his controversial work Hitler's Pope) cited this decision as evidence of his alleged silence toward anti-Semitism and The Holocaust. He utilized parts of it in his own inaugural encyclical Summi Pontificatus on the unity of human society, in October 1939, the month after the outbreak of World War II, and analysis of the draft figures prominently in most comparisons of the policies of Pius XII and his predecessor.
In June 2006, Pope Benedict XVI ordered all documents from the reign of Pius XI in the Vatican Secret Archives to be opened, and on September 18, 2006 over 30,000 documents were made available to researchers.
The first 70 paragraphs, probably authored by Gundlach, are a critique of theological modernism, unorthodox re-interpretation of Sacred Scripture, nationalism, and racism; the remaining 108, likely authored by all three Jesuits, contextualized the first part of the text with the social role of Catholic educational institutions and anti-Semitism.
Although the draft clearly condemned racism and anti-Semitism, the document is deeply grounded in anti-Judaism. The draft criticizes the majority of post-Messianic Jews for not acknowledging Jesus Christ as the true Jewish Messiah.
Humani generis unitas clearly condemns American racial segregation and racism and Nazi German anti-Semitism, though without explicitly naming these countries. Racism is a denial of the unity of human society, a denial of the human personality, and a denial of the true values of religion  There is no relation between race and religion, because racism is destructive to any society. Racism is destructive not only for social relations within a society but also for international relations and relations between different races.
The draft condemns the persecution of Jews. "These persecutions have been censured by the Holy See on more than one occasion, but especially when they have worn the mantle of Christianity". But the text hedges with an anti-Judaic theme. "This unjust and pitiless campaign against the Jews has at least this advantage", according to the draft, "because the true nature, the authentic basis of the social separation of the Jews from the rest of humanity ... is religious in character. ...Essentially, the so-called Jewish question ... is a question of religion and, since the coming of Christ, a question of Christianity." The encyclical blames the Jews of the time of Christ for having brought their own Messiah Jesus Christ to death: "The very act by which the Jewish people put to death their Savior and King was, in the strong language of Saint Paul, the salvation of the world." The draft continues by accusing Jews of blind materialism.
According to the authors, Pius XII was not aware of the text before the death of his predecessor. He chose not to publish it. However, his first encyclical Summi Pontificatus (On the Supreme Pontificate, October 12, 1939), published after the beginning of World War II, has an echo of the previous title in its title: On the Unity of Human Society and uses many of the arguments of the text, avoiding most of the negative characterization of the Jewish people and rabbinic religion contained in the text of the draft.
Summi Pontificatus sees Christianity as universalized and opposed to every form of racial hostility and every claim of racial superiority. There are no real racial differences: the human race forms a unity, because "one ancestor [God] made all nations to inhabit the whole earth".