14 A vast field for the apostolate has opened up on the national and international levels where the laity especially assist with their Christian wisdom. In loyalty to their country and in faithful fulfillment of their civic obligations, Catholics should feel themselves obliged to promote the true common good. Thus they should make the weight of their opinion felt in order that the civil authority may act with justice and that legislation may conform to moral precepts and the common good. Catholics skilled in public affairs and adequately enlightened in faith and Christian doctrine should not refuse to administer public affairs since by doing this in a worthy manner they can both further the common good and at the same time prepare the way for the Gospel. Catholics should try to cooperate with all men and women of good will to promote whatever is true, whatever just, whatever holy, whatever lovable (cf. Phil. 4:8). They should hold discussions with them, excel them in prudence and courtesy, and initiate research on social and public practices which should be improved in line with the spirit of the Gospel. Among the signs of our times, the irresistibly increasing sense of the solidarity of all peoples is especially noteworthy. It is a function of the lay apostolate sedulously to promote this awareness and to transform it into a sincere and genuine love of brotherhood. Furthermore, the laity should be aware of the international field and of the questions and solutions, doctrinal as well as practical, which arise in this field, with special reverence to developing nations. All who work in or give help to foreign nations must remember that relations among peoples should be a genuine fraternal exchange in which each party is at the same time a giver and a receiver. Travelers, whether their interest is international affairs, business, or leisure, should remember that they are itinerant heralds of Christ wherever they go and should act accordingly.
Source: Apostolicam Actuositatem (Vatican.va)