40 The Christian faithful, members of a living organism, cannot remain aloof and think that they have done their duty when they have satisfied their own spiritual needs; every individual must give his assistance to those who are working for the increase and propagation of God's kingdom. Our predecessor Pius XII reminded all of their common duty in these words: "A principal note of the Church is catholicity; consequently, a man is no true member of the Church unless he is likewise a true member of the entire body of Christian believers and is filled with an ardent desire to see her take root and flourish in every land." (70)
Source: Princeps Pastorum (Vatican.va)