Throughout the book so far, I have tried to set Jesus' earthly mission and message clearly against the background of the arriving kingdom of God to the earth. The first chapter showed how John the Baptist likely preached that God was about to come to judge the earth and restore Israel. The second chapter showed how Jesus' preaching of the coming kingdom fit right into this context. The third and fourth chapters talked about how Jesus' mission to the poor, the sick, and the demon possessed was in part about the restoration of all God's people, as well as the defeat of spiritual forces that stood in the way of God's rule.
This chapter is about how Jesus recruited others to participate in his mission. Here we especially think of his "disciples." The word "disciple" in Greek (mathētēs) is related to the word for learning (mathēteuō). So the gospels remember Jesus' followers as "learners" or as apprentices. As Jesus' apprentices, Jesus' disciples followed him around and learned his ways so that they could continue in his footsteps. They were Jesus' students who followed rabbi Jesus around and learned his ways and understanding of God.
Jesus had many followers. Stories about Jesus feeding five thousand and four thousand at a time make it clear that far more than twelve followed him around. At one point a group of men dig a hole in a roof to let a paralyzed man down because there was no room in the house (Mark 2:1-12). Soon Jesus stays out in deserted places because so many are following him (Mark 1:45; 3:7-9; 4:1; 5:24).
Among these many followers, Jesus picked twelve in particular. The gospels give slightly different names in their lists of who these twelve were (Mark 3:16-19; Matt. 10:2-4; Acts 1:13).  John never even gives a list and indicates that many of Jesus' followers left him at some point. John is very symbolic in its presentation and may be indicating that many of Jesus' early followers stopped following him because they could not accept his death and resurrection as part of the plan (John 6:53-66). 
It is less important to know the precise twelve as to notice the significance of the number twelve itself. In Matthew 19:28, Jesus tells the disciples that they will sit on twelve thrones judging the twelve tribes of Israel. This statement makes explicit something we might have already guessed, that the number twelve links Jesus' followers to the restoration of Israel. The fact that Jesus does not include himself within the number of the twelve puts him in a different category, namely, as messiah. ...
 The traditional way of handling the slight differences in the names of the twelve is to say that some of the disciples had more than one name, for example that Thaddeus in Matthew and Mark is Judas in Acts or that Levi in Mark 2:14 is Matthew. It is of course possible. From a standpoint of truth, it is preferable to live with the uncertainty. In real life, we would not harmonize details in this way but simply say that the sources do not completely agree on the precise names of the twelve.
 Many interpreters of John do not actually think that the "beloved disciple" who stands behind the Gospel of John was one of the twelve. Certainly the Gospel of John never identifies him as one of the twelve.
 So E. P. Sanders, Jesus and Judaism...