36 As members of the living Christ, incorporated into Him and made like unto Him through baptism and through confirmation and the Eucharist, all the faithful are duty - bound to cooperate in the expansion and spreading out of His Body, to bring it to fullness as soon as may be (Eph. 4:13). Therefore, all sons of the Church should have a lively awareness of their responsibility to the world; they should foster in themselves a truly catholic spirit; they should spend their forces in the work of evangelization. And yet, let everyone know that their first and most important obligation for the spread of the Faith is this: to lead a profoundly Christian life. For their fervor in the service of God and their charity toward others will cause a new spiritual wind to blow for the whole Church, which will then appear as a sign lifted up among the nations (cf. Is. 11:12), "the light of the world" (Matt. 5:14) and "the salt of the earth" (Matt. 5:13). This testimony of a good life will more easily have its effect if it is given in unison with other Christian communities, according to the norms of the Decree on Ecumenism, 12. From this renewed spirit, prayer and works of penance will be spontaneously offered to God that He may fructify the missionaries' work with His grace; and then there will be missionary vocations, and the material subsidies which the missions need will be forthcoming. But in order that each and every one of the Christian faithful may he fully acquainted with the present condition of the Church in the world, and may hear the voice of the multitudes who cry "Help us!" (cf. Acts 16:9), modern means of social communication should be used to furnish such mission information that the faithful may feel this mission work to be their very own, and may open their hearts to such vast and profound human needs, and may come to their assistance. It is also necessary to coordinate the information, and to cooperate with national and international agencies.
Source: Ad Gentes (Vatican.va)