Pope Pius XI, born Ambrogio Damiano Achille Ratti, was head of the Catholic Church from 6 February 1922 to his death in 1939. He was the first sovereign of Vatican City from its creation as an independent state on 11 February 1929. He assumed as his papal motto "Pax Christi in Regno Christi," translated "The Peace of Christ in the Kingdom of Christ."
Pius XI issued numerous encyclicals, including Quadragesimo anno on the 40th anniversary of Pope Leo XIII's groundbreaking social encyclical Rerum novarum, highlighting the capitalistic greed of international finance, the dangers of socialism/communism, and social justice issues, and Quas primas, establishing the feast of Christ the King in response to anti-clericalism. The encyclical Studiorum ducem, promulgated 29 June 1923, was written on the occasion of the 6th centenary 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".