18 Faith in Christ cannot maintain itself pure and unalloyed without the support of faith in the Church, "the pillar and ground of the truth" (1 Tim. iii. 15); for Christ Himself, God eternally blessed, raised this pillar of the Faith. His command to hear the Church ( Matt. xviii. 15), to welcome in the words and commands of the Church His own words and His own commands ( Luke x. 16), is addressed to all men, of all times and of all countries. The Church founded by the Redeemer is one, the same for all races and all nations. Beneath her dome, as beneath the vault of heaven, there is but one country for all nations and tongues; there is room for the development of every quality, advantage, task and vocation which God the Creator and Savior has allotted to individuals as well as to ethnical communities. The Church's maternal heart is big enough to see in the God-appointed development of individual characteristics and gifts, more than a mere danger of divergency. She rejoices at the spiritual superiorities among individuals and nations. In their successes she sees with maternal joy and pride fruits of education and progress, which she can only bless and encourage, whenever she can conscientiously do so. But she also knows that to this freedom limits have been set by the majesty of the divine command, which founded that Church one and indivisible. Whoever tampers with that unity and that indivisibility wrenches from the Spouse of Christ one of the diadems with which God Himself crowned her; he subjects a divine structure, which stands on eternal foundations, to criticism and transformation by architects whom the Father of Heaven never authorized to interfere.
Source: Mit Brennender Sorge (Vatican.va)