As interesting as it is to read about Jesus, it is not until chapter 4 that we read the words of Jesus himself. In the first part of the chapter, Jesus resisted the temptations of the devil; where Israel had failed in the desert, Jesus succeeded, and now he can move on to do what Israel was supposed to do - proclaim the word of God. Israel was meant to be the light for the world, the nation through which salvation would be extended to all, and now we have in Jesus a true Israelite to do what the whole nation was supposed to do.
The passage starts with a bit of narrative and a fulfilled quotation from Isaiah. I am a bit confused as to what to make of Jesus' retreat when John is arrested:
"Now when Jesus heard that John had been arrested, he withdrew to Galilee" (v.12)
I may have to think more about why the arrest of John has such an impact.
Next, I would like to urge any reader of the gospel of Matthew to think of many of the things that are "fulfilled" as affirmed in Jesus, more than open-ended prophecies which people are waiting for. Some really are that kind of prophecy - but a lot of the time we see things affirmed in Jesus as the true Christ, more than "this is a prediction which now comes true." Having said that, I am not quite sure under which category this quotation falls: affirmed or predicted-come-true. I suspect the former, just because of where the passage quoted in Isaiah is (the messianic section of Isaiah starts a lot later in the book).
"From that time Jesus began to proclaim, 'repent, for the kingdom of heaven is at hand." (v. 17)
We have seen how Jesus is being presented to us as the true king of Israel, having been given his Davidic lineage, been named the Messiah, given a miraculous birth, christened Son of God and excelled where Israel had fallen in temptation - now his ministry is going to begin to show this crucial fact. Jesus, King Jesus, has come to announce his kingdom. "The kingdom of heaven" is a foundational theme in the gospel according to St Matthew, and throughout the gospels the message of the King arriving constitutes the essence of what is meant by the "good news", or gospel.
walked by the Sea of Galilee, he saw two brothers, Simon, who is called
Peter, and Andrew his brother, casting a net into the lake—for they were
fishermen. And he said to them, ‘Follow me, and I will make you fish for people.’ Immediately they left their nets and followed him. As
he went from there, he saw two other brothers, James son of Zebedee and
his brother John, in the boat with their father Zebedee, mending their
nets, and he called them. Immediately they left the boat and their father, and followed him." (vv. 18-22)
This account is expanded in the gospel according to St Luke, but we shall get there when we do - it is important to understand passages first within the context of the book itself before venturing out to supplement from elsewhere. Jesus begins forming the inner circle of his group as he walks along the Sea of Galilee. He calls fishermen to follow him and transforms their vocation to the fishing of people. Now, fishermen were abundant at that time around the Sea of Galilee, so we are to understand the call of Simon (Peter) and Andrew, then the two sons of Zebedee, as the calling of ordinary people. Their response is quick and decisive: they respond to Jesus' call and follow "immediately" (vv. 20, 22).
We can nowadays be much more hesitant to respond to Jesus' call to follow him. These first disciples of Jesus, all four of which are saints of the Church and became the foundation of it, are ordinary people when Jesus calls them to be followers of his and fishers of people. They respond in the only way that is proper to respond to the call of God: faith. This passage expresses what it means to have faith in Jesus Christ - to come when he beckons and to step out in trust when there is not proof in the mathematical sense of Jesus' goodness. The responses of these first disciples - by no means perfect people - are an inspiration to me as I consider my own vocation, my own calling, because I think their response is the one Jesus wanted. So then, if I am to live consecrated to the Lord Jesus Christ, to whom I have already pledged all my life, I too must "immediately leave my place and follow him." I am called - yet it is not entirely clear what to. I think reflecting on the beatitudes tomorrow will do me good in that regard. Nonetheless, any who read the words of Jesus are also called to be his followers - we must come when he beckons, and go too where he commands.
"Jesus went throughout Galilee, teaching in their synagogues and proclaiming the good news of the kingdom and curing every disease and every sickness among the people. So
his fame spread throughout all Syria, and they brought to him all the
sick, those who were afflicted with various diseases and pains,
demoniacs, epileptics, and paralytics, and he cured them. And great crowds followed him from Galilee, the Decapolis, Jerusalem, Judea, and from beyond the Jordan." (vv. 23-25)
Jesus' ministry seems to have the following character: he travels, teaches and proclaims the kingdom of heaven (that is, he proclaims the gospel, ancient Greek for "good news"), and ministers to the sick. This is the model that I think many are called to follow, and the close relationship between proclamation of the gospel and works for the sick (or marginalized in general) is one that we sever to the detriment of both. It will become clearer why the two are so related soon in this gospel account.
Jesus gains himself some fame for these deeds, and people begin to come to him for healing. He gains himself quite the following from the surrounding area - he will soon preach the greatest sermon ever delivered to this crowd, and the message given is perhaps even more counter-cultural now than it was then. The crowds of people delighted that Jesus heals them are going to dwindle when it comes to responding to his call.
 For a treatment of this conception of the gospel in the writings of St Paul, see "What Saint Paul Really Said", by NT Wright. His treatment of St Paul the apostle is highly illuminating.
 I find the very idea that we can go and walk around the same place that Jesus did to this very day absolutely incredible. The mystery of the incarnation is indeed deep.
 See Ephesians 2:20.