Pius's pontificate was marked by great diplomatic activity and by many important papers, often in the form of encyclicals. In diplomatic affairs Pius was aided at first by Pietro Gasparri and after 1930 by Eugenio Pacelli (who succeeded him as Pius XII). Cardinal Gasparri's masterpiece was the Lateran Treaty (1929). Nevertheless, the Fascist government and the pope were in open disagreement over the restriction of youth activities; this culminated in a strong papal letter ( Non abbiamo bisogno, 1931), showing the impossibility of being at once a Fascist and a Catholic. Relations between Mussolini and the Holy See were cool ever after.
It fell to Cardinal Pacelli to negotiate a concordat for all Germany (1933). The Hitler government never pretended to observe the treaty. In 1937, after interference of every sort by the Nazis in Catholic life, the pope denounced the government and the Nationalist Socialist theory in a powerful encyclical, Mit brennender Sorge. A few days later he issued a definitive analysis of Communism from the Roman Catholic point of view in On Atheistic Communism. Pius also denounced persecutions in Russia, Mexico, and Spain. With England, the Netherlands, and France (where he condemned the royalist Action française movement in 1925) the pope was on unprecedentedly cordial terms. He spoke out continually against nationalism, racism, and totalitarianism and their menace to human dignity; hence the new feast of Christ the King, established to recall the rights of religion in the state, and hence, too, Pius's denunciation of anti-Semitism.
The pope, highly critical of laissez-faire capitalism, urged social reform especially in his encyclical Quadragesimo anno (1931), which renewed the plea made 40 years earlier by Leo XIII. Pius appealed directly to the laity for greater participation in all things religious—this he called Catholic Action. In the church's missionary activity he laid great stress on the necessity of integrating Christianity with native cultures rather than trying to make them European. This is seen in the Pontifical Work of St. Peter the Apostle for the Native Clergy, which he set up in 1929. To protect Catholics of Eastern rites from Latin influence he augmented the powers of their congregation and established a commission to study their canon law. He also called on Western Catholics to exhibit greater understanding of the Orthodox and other ancient churches of the East, notably in the encyclical Rerum Orientalium (1928).
Pius took delight in new technological developments. He established a broadcasting station at the Vatican and advanced the modernization of the Vatican Library. He also reconstituted the Pontifical Academy of the Sciences (1936), with a large international membership.