... continued from two months ago
The story of Acts begins with Jesus' ascension. Jesus has risen from the dead. He has appeared to his "apostles" for forty days and spoken about the kingdom of God. Then he gives them some final words before ascending to heaven.
There are many interesting things about these verses at the beginning of Acts (Acts 1:3-11). One of the first is that the disciples are called "apostles." To be sure, Luke calls the twelve disciples "apostles" several times in the Gospel of Luke. In that sense, it is not a new title for them in Luke-Acts, the name we give this two volume series of books in the New Testament. Jesus sends his disciples out even before his death and resurrection (e.g., Luke 9:10), so we can call them apostles even in Luke.
But there is also a sense in which we can say they do not fully become apostles until after Jesus' death and resurrection. An apostle is someone who is sent out as a messenger to represent some greater authority. In Acts, the apostles are especially witnesses to the fact that Jesus did indeed rise from the dead (e.g., Acts 1:8; 3:15). Paul confirms this understanding of an apostle when he says, "Am I not an apostle? Have I not seen Jesus our Lord?" (1 Cor. 9:1).
So we might say that the disciples--followers and apprentices of Jesus--technically only become "the apostles" after they witness Jesus' resurrection and are commissioned by him to go and proclaim that message throughout the world. In most of Acts, the apostles are twelve specific people, eleven of whom were key followers of Jesus while he was ministering in Galilee. There were Peter, James, John, and Andrew--perhaps apostles the earliest Christians considered to be pillars (cf. Gal. 2:9).  Then there were Philip and Thomas, Bartholomew and Matthew, James the son of Alphaeus and Simon the Zealot. If these are pairings by twos (cf. Luke 10:1), then Judas son of James was left without a mission partner. 
Judas of course betrays Jesus and the twelfth spot becomes vacant. One thing that takes place in the first chapter of Acts is Judas' replacement. They "cast lots" to decided between two candidates. While we do not know exactly what a "lot" was, the practice was something like drawing straws or rolling dice. While we would consider this sort of practice to be leaving things to chance, there was often a sense that God was directing the outcome (e.g., Prov. 16:33).
Someone named Matthias is chosen (Acts 1:26). You sometimes hear the suggestion that the replacement apostle should have really been the apostle Paul. After all, did not God apparently use him to spread the gospel more than any of the first eleven? Are there not thirteen books in the New Testament with his name on them?
Paul was sent by Jesus to witness the resurrection, but he did not fit the job description for a replacement of the Twelve, at least not the one that Peter gives in Acts 1:21-22. Judas' replacement needed to be someone who had followed Jesus from the baptism of John to the time of the ascension. Paul probably did not know Jesus until three years after he ascended to heaven.
This entire line of questioning brings up a very important question. To what extent are the biblical narratives description and to what extent are they prescription? ...
 Paul calls James, the brother of Jesus, a pillar in Acts 12:2. Perhaps he replaced James, the brother of John, after Herod Agrippa I put him to death (Acts 12:2).
 The lists differ slightly from gospel to gospel. Christian tradition has often dealt with this issue by saying, for example, that Thaddeus (Matt. 10:3) was the same as Judas son of James here. It is also possible, however, that the edges of Jesus' central circle were a little fuzzy.