From: Mark 16:9-15
Jesus Appears to Mary Magdalene and to Two Disciples
 Now when He (Jesus) rose early on the first day of the week, He appeared first to Mary Magdalene, from whom He had cast our seven demons.  She went and told those who had been with Him, as they mourned and wept.  But when they heard that He was alive and had been seen by her, they would not believe it.
 After this He appeared in another form to two of them, as they were walking into the country. And they went back and told the rest, but they did not believe them.
Jesus Appears to the Eleven. The Apostles' Mission
 Afterwards He appeared to the Eleven themselves as they sat at table; and He upbraided them for their unbelief and hardness of heart, because they had not believed those who saw Him after He had risen.  And He said to them, "Go into all the world and preach the Gospel to the whole creation."
11-14. When reporting these first appearances of the risen Jesus, St. Mark stresses the disciples' disbelief and their reluctance to accept the fact of the Resurrection, even though Jesus foretold it (cf. Mark 8:31; 9:31; 10:34). This resistance shown by the Apostles is a further guarantee of the truth of Jesus' resurrection; they were to be direct, specially-appointed witnesses to the risen Christ, yet they were reluctant to accept this role. They had personal, direct proof of the truth of the Resurrection.
However, our Lord will say: "Blessed are those who have not seen and yet believe" (John 20:29). In the Apostles' case, they needed, in addition to faith in the risen Christ, clear evidence of His resurrection, for they were to be the eye-witnesses, key witnesses who would proclaim it as an irrefutable fact. In this connection [Pope] St. Gregory the Great comments: "The reason why the disciples were slow to believe in the Resurrection was not so much due to their weakness as to our future firmness in the faith; what other purposes does this have (the very Resurrection being demonstrated by many arguments to those who were in doubt) than that our faith should be strengthened by their doubt?" ("In Evangelia Homilae", 16).
12. Our Lord's appearance to these two disciples is reported more fully by St. Luke (cf. 24:13-35).
15. This verse contains what is called the "universal apostolic mandate" (paralleled by Matthew 28:19-20 and Luke 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' (Ephesians 4:16). Between the members of this body there exists, further, such a unity and solidarity (cf. Ephesians 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).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.