Pius XI (born Achille Ratti May 31, 1857 - Rome, February 10, 1939) was Pope from February 6, 1922 until February 10, 1939.

He issued the encyclical Quas Primas establishing the feast of Christ the King. The main idea here is that the Catholic religion, beliefs, morality, and rule must spread itself to all areas of human living: the home, the city, politics, economics, art, etc.

Achille Ratti was born Desio, Province of Milan in 1857. An avid mountain climber and librarian, Ratti was the suprise choice of the 1922 papal conclave to replace Pope Benedict XV.

In 1929, the pope supervised the signing of the Lateran Treaties with Mussolini's Fascist government, which was intended to end the dispute that had existed between the Roman Catholic Church and the Kingdom of Italy over the former Papal States, and in particular Rome, all of which had been seized by the forces of King Victor Emmanuel II of Piedmont in 1870 at the foundation of the modern unified Italian state. According to the terms of the treaty, Vatican City was given sovereignty as an enclave of the city of Rome in return for the Vatican relinquishing its claim to the former territories of the Papal States. Pope Pius thus became head of state, the first pope who could be termed as such since the Papal States fell after the unification of Italy in the 19th century. The relationship to Mussolini's government deteriorated drastically in the following years. As a consequence Pius issued the encyclical Non Abbiamo Bisogno (1931). In 1937 he issued the encyclical Mit Brennender Sorge with condemnation of Nazi ideology of racism.

The concordat also entailed an agreement with Italy that provided for monies being transferred to the Church to aid with the transition and intended as a compensation for the loss of the territories laid claim to by the Church (estimated to be around 700 million Lire). During the reign of Pope Pius XI this money was used for investments in the stock markets and real estate that laid the foundation of the modern wealth of the church. To manage these investments, the Pope appointed the lay-person Bernadino Nogara, who through shrewd investing in stocks, gold, and futures markets, vastly increased the Catholic Church's financial holdings.