Given the rank of archbishop, Costantini reached China in late 1922 and immediately began to push for implementation of the directives outlined by Pope Benedict XV in his missionary encyclical Maximum illud (1919). In the so-called First Council of China, held in Shanghai from May 15 to June 13, 1924, he convened all the foreign heads of Catholic missions in China, delegates of various missionary institutes, and representatives of the Chinese clergy to plan a general reform of the Catholic Church in China. Together they addressed pressing problems, such as the tension between native and foreign priests, the promotion of the Chinese clergy and the ordination of Chinese bishops, the creation of new commissions for the work of the apostolate, and the liberation of the church from the political influence of the French protectorate.
Despite resistance by some foreign bishops, Costantini's relentless efforts during his 13-year term in China produced good results. Missionary training began to emphasize understanding of and respect for Chinese culture. Chinese priests in increasing number acceded to important positions. When Costantini arrived in China, the Catholic Church was under foreign missionary control; by the time the 1924 council met, three prefectures apostolic were already headed by Chinese prelates.
Two years later, in October 1926, Pope Pius XI ordained six Chinese bishops (the first such ordinations since 1685). By 1933, when Costantini finished his mandate, 19 of the existing 119 ecclesiastical territories were in Chinese hands. The prelate also contributed to the idea that the Chinese had primary responsibility for converting their own people when he founded, in 1926, the Disciples of the Lord, a Chinese religious congregation for the purpose of imparting the missionary spirit to the local clergy. Costantini, who had a deep appreciation for the sacred arts, strongly advocated the development of Chinese forms of expression.
Through his influence, Chinese-style buildings, statues, images, and music began to gain acceptance in the Catholic Church. From 1935 to 1953, Costantini was secretary of Propaganda Fide in Rome. In 1953 Pope Pius XII made him a cardinal. Till his death, Costantini remained a champion of missionary accommodation and of local churches rooted in their own cultures and headed by native bishops. He succeeded in helping the Holy See free itself from the protectorate system and its nationalistic implications.