34 Missionary activity proper, namely the mission ad gentes , is directed to "peoples or groups who do not yet believe in Christ," "who are far from Christ," in whom the Church "has not yet taken root" and whose culture has not yet been influenced by the Gospel. It is distinct from other ecclesial activities inasmuch as it is addressed to groups and settings which are non-Christian because the preaching of the Gospel and the presence of the Church are either absent or insufficient. It can thus be characterized as the work of proclaiming Christ and his Gospel, building up the local Church and promoting the values of the kingdom. The specific nature of this mission ad gentes consists in its being addressed to "non-Christians." It is therefore necessary to ensure that this specifically "missionary work that Jesus entrusted and still entrusts each day to his Church" does not become an indistinguishable part of the overall mission of the whole People of God and as a result become neglected or forgotten. On the other hand, the boundaries between pastoral care of the faithful, new evangelization and specific missionary activity are not clearly definable, and it is unthinkable to create barriers between them or to put them into watertight compartments. Nevertheless, there must be no lessening of the impetus to preach the Gospel and to establish new churches among peoples or communities where they do not yet exist, for this is the first task of the Church, which has been sent forth to all peoples and to the very ends of the earth. Without the mission ad gentes , the Church's very missionary dimension would be deprived of its essential meaning and of the very activity that exemplifies it. Also to be noted is the real and growing interdependence which exists between these various saving activities of the Church. Each of them influences, stimulates and assists the others. The missionary thrust fosters exchanges between the churches and directs them toward the larger world, with positive influences in every direction. The churches in traditionally Christian countries, for example, involved as they are in the challenging task of new evangelization, are coming to understand more clearly that they cannot be missionaries to non-Christians in other countries and continents unless they are seriously concerned about the non-Christians at home. Hence missionary activity ad intra is a credible sign and a stimulus for missionary activity ad extra , and vice versa.
Source: Redemptoris Missio (Vatican.va)