48 Conversion and Baptism give entry into a Church already in existence or require the establishment of new communities which confess Jesus as Savior and Lord. This is part of God's plan, for it pleases him "to call human beings to share in his own life not merely as individuals, without any unifying bond between them, but rather to make them into a people in which his children, who had been widely scattered, might be gathered together in unity." The mission ad gentes has this objective: to found Christian communities and develop churches to their full maturity. This is a central and determining goal of missionary activity, so much so that the mission is not completed until it succeeds in building a new particular church which functions normally in its local setting The Decree Ad Gentes deals with this subject at length, and since the Council, a line of theological reflection has developed which emphasizes that the whole mystery of the Church is contained in each particular church, provided it does not isolate itself but remains in communion with the universal Church and becomes missionary in its own turn. Here we are speaking of a great and lengthy process, in which it is hard to identify the precise stage at which missionary activity properly so-called comes to an end and is replaced by pastoral activity. Even so, certain points must remain clear.
Source: Redemptoris Missio (Vatican.va)