The text consists of two parts: a summary of Jesus' preaching in 1:14-15; and a call story in 1:16-20.
These two parts are connected by the sense of urgency brought on via the proclamation of the reign of God.
Mark tells us that after John was "handed over" (paradothēnai), Jesus returned to Galilee and began to preach the gospel. The NRSV translates paradothēnai as "arrested," however, the Greek verb has a much fuller sense. It includes an allusion to John's death, for the verb paradidōmi is used in Mark and elsewhere in the New Testament to speak of Jesus' being handed over to death (e.g., Mark 14:21; Romans 4:25; 1 Corinthians 11:23). Consider the evidence linking these two men's deaths. Both men were prophets who offended the powers that be. Both died violent deaths. Moreover, Mark placed similar passages questioning the identity of Jesus before the account of John's death (6:14-16), and before the first of the three passion predictions (8:27-30), which suggests that he saw a parallel between the two men's deaths. As has been long observed, the cross of Jesus casts a long shadow over the Gospel of Mark. Thus, already in Mark 1:14 the mention of John's being "handed over" raises the specter of Jesus' death. For Mark, Jesus' kingdom ministry takes place, from the very beginning, under the shadow of the cross.
In 1:15 there follows a summary of Jesus' preaching. Such summaries are common throughout the synoptic gospels (cf. Matthew 4:23; 9:35). Mark appears to have constructed this summary on material from Jesus' preaching. The very fact that Mark places this summary of Jesus' preaching of the gospel after mention of John's being "handed over" may be based on tradition containing Jesus' view of history such as is recorded in Luke 16:16, namely, that "the law and the prophets were in effect until John came; since then the good news of the kingdom of God is proclaimed." With the end of John's ministry comes the end of one stage of history. Now, Jesus and his gospel come to center stage. Jesus' ministry is the center of all history.
The reliance on traditional material is evident in the rest of 1:15. The proclamation that "the kingdom of God has come near" (ēggiken hē basileia tou theou) is found elsewhere in Jesus' sayings (Matthew 10:7; Luke 10:9, 11). In addition, Jesus' healings and exorcisms were particularly connected to the coming of the kingdom (Matthew 12:28, Luke 11:20; cf. Matthew 10:7-8, Luke 10:9). It is unnecessary to enter the old debate of whether Jesus meant that the kingdom of God had actually come (realized eschatology), or whether the kingdom of God was near but not yet here (future eschatology). It is possible that Jesus thought that both were true. Wherever he conducted his ministry, there God's reign was actively coming into being, even if the kingdom might not come fully until the future.
The announcement that "the time (kairos) is fulfilled (peplērōtai)" also has the ring of tradition. Luke similarly begins his account of Jesus' Galilean ministry with Jesus preaching in his hometown synagogue in Nazareth about the "year of the Lord's favor" that Isaiah had prophesied (Isaiah 61:2). Jesus then said, that time had been fulfilled (peplērōtai) in the people's hearing that very day (Luke 4:19, 21).
Interestingly, Isaiah 60:22, coming just before the part of Isaiah quoted in Luke 4, speaks of the time (Greek: kairos) when God would bring about the restoration of Israel. Possibly this is the source of the statement about the kairos in Mark 1:15. It is likely that the latter half of Isaiah lies behind Jesus' eschatological vision, and is the source of his understanding of the kingdom. Isaiah 61:1 characterizes the time of the Lord's favor as a time of preaching the good news (Greek: euaggelizesthai). Isaiah 52:7 connects the preaching of good news with the proclamation of God's reign. Thus, the good news of the kingdom of God is that the one true God, with his life and peace and truth, is about to establish his rule over the world. All other opposing powers--whether human powers or sin or evil or death--are destined to end their rule.
We have seen that the announcement that the kingdom of God is near also appears in Matthew 10:7 and Luke 10:9, 11. Closely connected with this announcement in the tradition known to Luke (a so-called Q tradition), was the call to repentance (Luke 10:13). Perhaps the juxtaposition of the announcement of the reign of God and the call to repentance in the summary of Mark 1:15 is rooted in similar traditional material. In any case, the theological basis for the juxtaposition is clear. Announcing that God's reign is near has the consequence of an urgent call for repentance, that is, aligning one's values and way of life with God's ways. In today's epistle reading (1 Corinthians 7:29-31) Paul similarly calls for an examination of our priorities in light of the kairos.
The second section (Mark 1:16-20) then illustrates what the urgent call of the kingdom looks like. Jesus, walking along the Sea of Galilee, sees the two brothers Simon and Andrew, fishermen, casting their nets in the sea. He calls them to follow, and immediately, in obedience, they leave their nets and follow him. The same happens with James and John. The kai euthys ("and immediately") of 1:18 and 1:20, a favorite turn of phrase of Mark, gives expression to the urgency of the call. The time is here, God's kingdom is near; there is no time to lose!
It is striking that these four men would drop everything to follow Jesus if they did not already know him. Indeed, some scholars have speculated that they actually knew Jesus, or knew about him, before he called them into discipleship (cf. John 1:35-40). Whatever the history of the relationship between Jesus and these four men may have been, however, the story gives effective expression to the urgency of the call to discipleship.
Consider also that Mark portrays Jesus' proclamation of the kingdom coming not only as a gift ("good news"), but also with a demand ("repent"). Writing as a Lutheran, I find this a salutary warning that we dare not limit the force of Jesus' preaching of the kingdom by imposing upon it a rigid Law-Gospel grid. If we try to impose such a grid on his preaching and teaching, we will not understand them in their integrity. To be sure, the indicative ("the time is fulfilled, the kingdom of God has come near") precedes the imperative ("repent and believe in the good news"). What humans do comes as a response to God's prevenient action. Still, we must not overlook that the one who promises the kingdom to sinners is the same one who calls sinners to repentance and who calls disciples to give up all that they have to follow him. To put it in Pauline terms, Christ is my life (cf. Philippians 1:21); my life is a total gift from him. Therefore I must also be willing and prepared to forfeit everything for him (Philippians 3:7-8; cf. Mark 8:35). Since in Jesus everything is given to me, in Jesus everything is demanded of me. The four disciples' willingness to throw in their lot completely with Jesus illustrates that attitude.