9 And so the time for missionary activity extends between the first coming of the Lord and the second, in which latter the Church will be gathered from the four winds like a harvest into the kingdom of God. For the Gospel must be preached to all nations before the Lord shall come (cf. Mark 13:10). Missionary activity is nothing else and nothing less than an epiphany, or a manifesting of God's decree, and its fulfillment in the world and in world history, in the course of which God, by means of mission, manifestly works out the history of salvation. By the preaching of the word and by the celebration of the sacraments, the center and summit of which is the most holy Eucharist, He brings about the presence of Christ, the author of salvation. But whatever truth and grace are to be found among the nations, as a sort of secret presence of God, He frees from all taint of evil and restores to Christ its maker, who overthrows the devil's domain and wards off the manifold malice of vice. And so, whatever good is found to be sown in the hearts and minds of men, or in the rites and cultures peculiar to various peoples, not only is not lost, but is healed, uplifted, and perfected for the glory of God, the shame of the demon, and the bliss of men. Thus, missionary activity tends toward eschatological fullness. For by it the people of God is increased to that measure and time which the Father has fixed in His power(cf. Acts 1:7). To this people it was said in prophecy: "Enlarge the space for your tent, and spread out your tent cloths unsparingly" (Is. 54:2). By missionary activity, the mystical body grows to the mature measure of the fullness of Christ (cf. Eph. 4:13); and the spiritual temple, where God is adored in spirit and in truth (cf. John 4:23), grows and is built up upon the foundation of the Apostles and prophets, Christ Jesus Himself being the supreme corner stone (Eph. 2:20).
Source: Ad Gentes (Vatican.va)