18 The Church will certainly not refuse her zealous aid to States united under the Christian law in any of their undertakings inspired by justice and charity, inasmuch as she is herself the most perfect type of universal society. She possesses in her organization and institutions a wonderful instrument for bringing this brotherhood among men, not only for their eternal salvation but also for their material well-being to the sure acquisition of eternal blessings. It is the teaching of history that when the Church pervaded with her spirit the ancient and barbarous nations of Europe, little by little the many and varied differences that divided them were diminished and their quarrels extinguished; in time they formed a homogeneous society from which sprang Christian Europe which, under the guidance and auspices of the Church, whilst preserving a diversity of nations, tended to a unity that favoured its prosperity and glory. On this point St. Augustine well says: "This celestial city, in its life here on earth, calls to itself citizens of every nation, and forms out of all the peoples one varied society; it is not harassed by differences in customs, laws and institutions, which serve to attainment or the maintenance of peace on earth; it neither rends nor destroys anything but rather guards all and adapts itself to all; however these things may vary among the nations, they are all directed to the same end of peace on earth as long as they do not hinder the exercise of religion, which teaches the worship of the true supreme God." And the same holy Doctor thus addresses the Church: "Citizens, peoples and all men, thou, recalling their common origin, shalt not only unite among themselves, but shalt make them brothers."
Source: Pacem, Dei Munus Pulcherrimum (Vatican.va)