From: Mark 16:15-20
Jesus Appears to the Eleven. The Apostle's Mission
 And He (Jesus) said to them (the Eleven), "Go into all the world and preach the Gospel to the whole creation.  He who believes and is baptized will be saved; but he who does not believe will be condemned.  And these signs will accompany those who believe; in My name they will cast out demons; they will speak in new tongues;  they will pick up serpents, and if they drink any deadly thing, it will not hurt them; they will lay their hands on the sick, and they will recover."
 So then the Lord Jesus, after He had spoken to them, was taken up into Heaven, and sat down at the right hand of God.
The Apostles Go Forth and Preach
 And they went forth and preached everywhere, while the Lord worked with them and confirmed the message by the signs that attended it. Amen.
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).
16. This verse teaches that, as a consequence of the proclamation of the Good News, faith and Baptism are indispensable pre-requisites for attaining salvation. Conversion to the faith of Jesus Christ should lead directly to Baptism, which confers on us "the first sanctifying grace, by which Original Sin is forgiven, and which also forgives any actual sins there may be; it remits all punishment due for sins; it impresses on the soul the mark of the Christian; it makes us children of God, members of the Church and heirs to Heaven, and enables us to receive the other Sacraments" ("St. Pius X Catechism", 553).
Baptism is absolutely necessary for salvation, as we can see from these words of the Lord. But physical impossibility for receiving the rite of Baptism can be replaced either by martyrdom (called, therefore, "baptism of blood") or by a perfect act of love of God and of contrition, together with an at least implicit desire to be baptized: this is called "baptism of desire" (cf. "ibid.", 567-568).
Regarding infant Baptism, St. Augustine taught that "the custom of our Mother the Church of infant Baptism is in no way to be rejected or considered unnecessary; on the contrary, it is to be believed on the ground that it is a tradition from the Apostles" ("De Gen., Ad Litt.", 10, 23, 39). The new "Code of Canon Law" also stresses the need to baptize infants: "Parents are obliged to see that their infants are baptized within the first few weeks. As soon as possible after the birth, indeed even before it, they are to approach the parish priest to ask for the Sacrament for their child, and to be themselves duly prepared for it" (Canon 867).
Another consequence of the proclamation of the Gospel, closely linked with the previous one, is that "the Church is necessary", as Vatican II declares: "Christ is the one mediator and way of salvation; He is present to us in His body which is the Church. He Himself explicitly asserted the necessity of faith and Baptism (cf. Mark 16:16; John 3:5), and thereby affirmed at the same time the necessity of the Church which men enter through Baptism as through a door. Hence they could not be saved who, knowing that the Church was founded as necessary by God through Christ, would refuse to enter it, or to remain in it" ("Lumen Gentium", 14; cf. "Presbyterorum Ordinis", 4; "Ad Gentes", 1-3; "Dignitatis Humanae", 11).
17-18. In the early days of the Church, public miracles of this kind happened frequently. There are numerous historical records of these events in the New Testament (cf., e.g., Acts 3:1-11; 28:3-6) and in other ancient Christian writings. It was very fitting that this should be so, for it gave visible proof of the truth of Christianity. Miracles of this type still occur, but much more seldom; they are very exceptional. This, too, is fitting because, on the one hand, the truth of Christianity has been attested to enough; and, on the other, it leaves room for us to merit through faith. St. Jerome comments: "Miracles were necessary at the beginning to confirm the people in the faith. But, once the faith of the Church is confirmed, miracles are not necessary" ("Comm. In Marcum, in loc."). However, God still works miracles through saints in every generation, including our own.
19. The Lord's ascension into Heaven and His sitting at the right hand of the Father is the sixth article of faith confessed in the Creed. Jesus Christ went up into Heaven body and soul, to take possession of the Kingdom He won through His death, to prepare for us a place in Heaven (cf. Revelation 3:21) and to send the Holy Spirit to His Church (cf. "St. Pius X Cathechism", 123).
To say that He "sat at the right hand of God" means that Jesus Christ, including His humanity, has taken eternal possession of Heaven and that, being the equal of His Father in that He is God, He occupies the place of highest honor beside Him in His human capacity (cf. "St. Pius V Catechism", I, 7, 2-3). Already in the Old Testament the Messiah is spoken of as seated at the right hand of the Almighty, thereby showing the supreme dignity of Yahweh's Annointed (cf. Psalm 110:1). The New Testament records this truth here and also in many other passages (cf. Ephesians 1:20-22; Hebrews 1:13).
As the "St. Pius V Catechism" adds, Jesus went up to Heaven by His own power and not by any other. Nor was it only as God that He ascended, but also as man.
20. Inspired by the Holy Spirit, the evangelist attests that the words of Christ have already begun to be fulfilled by the time of writing. The Apostles, in other words, were faithfully carrying out the mission of our Lord entrusted to them. They begin to preach the Good News of salvation throughout the known world. Their preaching was accompanied by the signs and wonders the Lord had promised, which lent authority to their witness and their teaching. Yet, we know that their apostolic work was always hard, involving much effort, danger, misunderstanding, persecution and even martyrdom--like our Lord's own life.
Thanks to God and also to the Apostles, the strength and joy of our Lord Jesus Christ has reached as far as us. But every Christian generation, every man and woman, has to receive the preaching of the Gospel and, in turn, pass it on. The grace of God will always be available to us: "Non est abbreviata manus Domini" (Isaiah 59:1), the power of the Lord has not diminished.
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.