From: Mark 3:13-19
Jesus Chooses Twelve Apostles
 And He (Jesus) went up into the hills, and called to Him those whom He desired; and they came to Him.  And He appointed twelve, to be with Him, and to be sent out to preach  and have authority to cast out demons;  Simon whom He surnamed Peter;  James the son of Zebedee and John the brother of James, whom He surnamed Boanerges, that is, sons of thunder;  Andrew, and Philip, and Bartholomew, and Matthew, and Thomas, and James the son of Alphaeus, and Thaddaeus, and Simon the Cananaean,  and Judas Iscariot, who betrayed Him.
13. "He called to Him those whom He desired": God wants to show us that calling, vocation, is an initiative of God. This is particularly true in the case of the Apostles, which is why Jesus could tell them, later on, that "you did not choose Me, but I chose you" (Jn 15:16). Those who will have power and authority in the Church will not obtain this because first they offer their services and then Jesus accepts their offering: on the contrary, "not through their own initiative and preparation, but rather by virtue of divine grace, would they be called to the apostolate" (St. Bede, "In Marci Evangelium Expositio, in loc.").
14-19. The Twelve chosen by Jesus (cf. 3:14) receive a specific vocation to be "people sent out", which is what the word "apostles" means. Jesus chooses them for a mission which He will give them later (6:6-13) and to enable them to perform this mission He gives them part of His power. The fact that He chooses "twelve" is very significant. This is the same number as the twelve Patriarchs of Israel, and the Apostles represent the new people of God, the Church founded by
Christ. Jesus sought in this way to emphasize the continuity that exists between the Old and New Testaments. The Twelve are the pillars on which Christ builds His Church (cf. Gal 2:9); their mission to make disciples of the Lord (to teach) all nations, sanctifying and governing the believers (Mt 28:16-20; Mk 16:15; Lk 24:45-48; Jn 20:21-23).
14. The Second Vatican Council sees in this text the establishment of the College of the Apostles: "The Lord Jesus, having prayed at length to the Father, called to Himself those whom He willed and appointed twelve to be with Him, whom He might send to preach the Kingdom of God (cf. Mk 3:13-19; Mt 10:1-42). These apostles (cf. Lk 6:13) He constituted in the form of a college or permanent assembly, at the head of which He placed Peter, chosen from amongst them" (cf. Jn 21: 15-17) [...]. "That divine mission, which was committed by Christ to the apostles, is destined to last until the end of the world (cf. Mt 28:20), since the Gospel, which they were charged to hand on, is, for the Church, the principle of all its life for all time. For that very reason the apostles were careful to appoint successors in their hierarchically constituted society." ("Lumen Gentium", 19-20). Therefore, the Pope and the bishops, who succeed to the College of the Twelve, are also called by our Lord to be always with Jesus and to preach the Gospel, aided by priests.
Life in union with Christ and apostolic zeal must be very closely linked together; in other words, effectiveness in apostolate always depends on union with our Lord, on continuous prayer and on sacramental life: "Apostolic zeal is a divine craziness I want you to have. Its symptoms are: hunger to know the Master; constant concern for souls; perseverance that nothing can shake" (St J. Escriva, "The Way", 934).
16. At this point, before the word "Simon" the sentence "He formed the group of the twelve" occurs in many manuscripts (it is similar to the phrase "He appointed twelve" in v. 14) but it is not included in the New Vulgate. The repetition of the same _expression and the article in "the twelve" show the importance of the establishment of the Apostolic College.
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.