He said to them, “Go into all the world and preach the gospel to all creation. Whoever believes and is baptized will be saved, but whoever does not believe will be condemned. And these signs will accompany those who believe: In my name they will drive out demons; they will speak in new tongues; they will pick up snakes with their hands; and when they drink deadly poison, it will not hurt them at all; they will place their hands on sick people, and they will get well.”

Mark 16: 15-18

Jesus appears to the Eleven. The Apostles’ mission.
This verse contains what is called the “universal apostolic mandate” (paralleled by Mt 28:19-20 and Lk 24:46-48). This is an imperative command from Christ to his Apostles to preach the Gospel to the whole world. This same apostolic mission applies, especially, to the Apostles’ successors, the Bishops in communion with Peter’s successor, the Pope.

But this mission extends further: the whole “Church was founded to spread the kingdom of Christ over all the earth for the glory of God the Father, to make all men partakers in redemption and salvation ... Every activity of the Mystical Body with this in view goes by the name of ‘apostolate’; the Church exercises it through all its members, though in various ways. In fact, the Christian vocation is, of its nature, a vocation to the apostolate as well”. In the organism of a living body no member plays a purely passive part, sharing in the life of the body it shares at the same time in its activity. The same is true for the Body of Christ, the Church: ‘the whole Body achieves full growth in dependence on the full functioning of each part’ (Eph 4:16). Between the members of this body there exists, further, such a unity and solidarity (cf. Eph 4:16) that a member who does not work at the growth of the body to the extent of his possibilities must be considered useless both to the Church and to himself.

“In the Church there is diversity of ministry but unity of mission. To the apostles and their successors Christ has entrusted the office of teaching, sanctifying and governing in his name and by his power. But the laity are made to share in the priestly, prophetical and kingly office of Christ; they have therefore, in the Church and in the world, their own assignment in the mission of the whole People of God” (Vatican II, Apostolicam actuositatem, 2).

It is true that God acts directly on each person’s soul through grace, but it must also be said that it is Christ’s will (expressed here and elsewhere) that men should be an instrument or vehicle of salvation for others.

Vatican II also teaches this: “On all Christians, accordingly, rests the noble obligation of working to bring all men throughout the whole world to hear and accept the divine message of salvation” (ibid., 3).

Catechism

748

"Christ is the light of humanity; and it is, accordingly, the heart-felt desire of this sacred Council, being gathered together in the Holy Spirit, that, by proclaiming his Gospel to every creature, it may bring to all men that light of Christ which shines out visibly from the Church." These words open the Second Vatican Council's Dogmatic Constitution on the Church. By choosing this starting point, the Council demonstrates that the article of faith about the Church depends entirely on the articles concerning Christ Jesus. The Church has no other light than Christ's; according to a favorite image of the Church Fathers, the Church is like the moon, all its light reflected from the sun.