Pius XI (Latin: Pius PP. XI; Italian: Pio XI), born Ambrogio Damiano Achille Ratti (Italian pronunciation: [amˈbrɔdʒo daˈmjano aˈkille ˈratti]; 31 May 1857 – 10 February 1939), was Pope from 6 February 1922, and sovereign of Vatican City from its creation as an independent state on 11 February 1929 until his death on 10 February 1939. He took as his papal motto, "Pax Christi in Regno Christi," translated "The Peace of Christ in the Reign of Christ."

Pope Pius XI issued numerous encyclicals including Quadragesimo Anno on the 40th anniversary of Pope Leo XIII's groundbreaking social encyclical Rerum Novarum, highlighting capitalistic greed of international finance, and social justice issues, and Quas Primas, establishing the feast of Christ the King. The encyclical Studiorum ducem, promulgated the 29 June 1923, was written on the occasion of the 6th centennary of the canonization of Thomas Aquinas, whose thought is acclaimed as central to Catholic philosophy and theology. The encyclical also singles out the Pontifical University of Saint Thomas Aquinas, Angelicum as the preeminent institution for the teaching of Aquinas: "ante omnia Pontificium Collegium Angelicum, ubi Thomam tamquam domi suae habitare dixeris," (before all others the Pontifical Angelicum College, where Thomas can be said to dwell).