6 This duty, to be fulfilled by the order of bishops, under the successor of Peter and with the prayers and help of the whole Church, is one and the same everywhere and in every condition, even though it may be carried out differently according to circumstances. Hence, the differences recognizable in this, the Church's activity, are not due to the inner nature of the mission itself, but rather to the circumstances in which this mission is exercised. These circumstances in turn depend sometimes on the Church, sometimes on the peoples or groups or men to whom the mission is directed. For the Church, although of itself including the totality or fullness of the means of salvation, does not and cannot always and instantly bring them all into action. Rather, she experiences beginnings and degrees in that action by which she strives to make God's plan a reality. In fact, there are times when, after a happy beginning, she must again lament a setback, or at least must linger in a certain state of unfinished insufficiency. As for the men, groups and peoples concerned, only by degrees does she touch and pervade them, and thus take them up into full catholicity. The right sort of means and actions must be suited to any state or situation. "Missions" is the term usually given to those particular undertakings by which the heralds of the Gospel, sent out by the Church and going forth into the whole world, carry out the task of preaching the Gospel and planting the Church among peoples or groups who do not yet believe in Christ. These undertakings are brought to completion by missionary activity and are mostly exercised in certain territories recognized by the Holy See. The proper purpose of this missionary activity is evangelization, and the planting of the Church among those peoples and groups where it has not yet taken root. Thus from the seed which is the word of God, particular autochthonous churches should be sufficiently established and should grow up all over the world, endowed with their own maturity and vital forces. Under a hierarchy of their own, together with the faithful people, and adequately fitted out with requisites for living a full Christian life, they should make their contribution to the good of the whole Church. The chief means of the planting referred to is the preaching of the Gospel of Jesus Christ. To preach this Gospel the Lord sent forth His disciples into the whole world, that being reborn by the word of God (cf. 1 Peter 1:23), men might be joined to the Church through baptism - that Church which, as the body of the Word Incarnate, is nourished and lives by the word of God and by the eucharistic bread (cf. Acts 2:43). In this missionary activity of the Church various stages sometimes are found side by side: first, that of the beginning or planting, then that of newness or youth. When these have passed, the Church's missionary activity does not cease, but there lies upon the particular churches already set up the duty of continuing this activity and of preaching the Gospel to those still outside. Moreover, the groups among which the Church dwells are often radically changed, for one reason or other, so that an entirely new set of circumstances may arise. Then the Church must deliberate whether these conditions might again call for her missionary activity. Besides, circumstances are sometimes such that, for the time being, there is no possibility of expounding the Gospel directly and forthwith. Then, of course, missionaries can and must at least bear witness to Christ by charity and by works of mercy, with all patience, prudence and great confidence. Thus they will prepare the way for the Lord and make Him somehow present. Thus it is plain that missionary activity wells up from the Church's inner nature and spreads abroad her saving Faith. It perfects her Catholic unity by this expansion. It is sustained by her apostolicity. It exercises the collegial spirit of her hierarchy. It bears witness to her sanctity while spreading and promoting it. Thus, missionary activity among the nations differs from pastoral activity exercised among the faithful as well as from undertakings aimed at restoring unity among Christians. And yet these two ends are most closely connected with the missionary zeal because the division among Christians damages the most holy cause of preaching the Gospel to every creature and blocks the way to the faith for many. Hence, by the very necessity of mission, all the baptized are called to gather into one flock, and thus they will be able to bear unanimous witness before the nations to Christ their Lord. And if they are not yet capable of bearing witness to the same faith, they should at least be animated by mutual love and esteem.
Source: Ad Gentes (Vatican.va)