After a brief description of Jesus' temptation in the Desert (Mk. 1:12-13), Mark immediately proceeds to narrate the beginnings of Jesus' ministry (Mk. 1:14-15) and the call of the first disciples (Mk. 1:16-20). Matthew for his part, while retaining the Marcan outline builds up the narrative with his own materials and in so doing adds more nuances to the account of the beginning of Jesus' ministry (Mt. 4:12-17), and the call of the fishermen (Mt. 4:18-22). The liturgy of the third Sunday in Ordinary Time (Year A) adds the beginning of the summary section of the healing activity of Jesus (4:23-25) which was intended to explain the "crowd" in Mt. 5:1, the audience of the Sermon on the Mount (Mt. 5:1-7:29).
Matthew writes that Jesus returned to Galilee after hearing of the Baptist's arrest. He was after all in the wilderness of Judea for a time (cf. Mt. 4:1-11). Then using two participles ("leaving ... coming"), Matthew tells us that Jesus transferred from Nazareth to Caphernaum in order to dwell there. The evangelist underlines the fact Caphernaum is by the sea, anticipating the following section where Jesus comes into contact with fishermen. Matthew explains that Jesus' "appearance" in this region of Galilee is fulfillment of the prophecy in Isaiah 9:1-2.
Isaiah 9:1-7 is an oracle about the reversal of fortunes of the territories north of Judah that Tiglath Pileser III had turned into Assyrian provinces after the capture of the Northern Israelite capital, Samaria (721 BC). Zebulun and Naphtali became the Assyrian Province of Galilee, the Way by the Sea is Dor and "beyond the Jordan" is Gilead. In this oracle, the birth of a new king (Hezekiah?) announces that process of reversal. The light that shines and breaks the gloomy and deathly darkness is the new boy king. Matthew no doubt understood that the light is Christ who now walks in the land that used to be Zebulun and Naphtali. He has prepared his audience for this connection between "light" and "Jesus" in his narration of the Magis' visit (2:1ff). He will again build on this connection with the saying on the "light of the eyes" (Mt. 6:23). For the moment, the figure of Jesus is the concern who now begins his ministry with the same words that characterized the preaching of the Baptist: "Repent, for the kingdom of heaven is at hand." (Mt. 4:17; cf. 3:1).
Taken out of context, this summary statement about Jesus' message may sound as if Jesus was just a Baptist disciple continuing the work of John. But seeing how John talked about him and how he regarded him should be reason enough for us to understand 4:17 and even 3:1 as summaries. Both Jesus and John called for repentance; both of them proclaimed the kingdom's nearness. But Jesus is "the One Mightier" than John and he will be one who will gather in people for the kingdom as the following section will show.
The call of the four disciples closely follow along Marcan lines. The pattern is patent. There are two sections which narrate the call of two pairs of brothers, all fishermen. Both begin with Jesus walking along and "seeing" them. Both end with a statement of the brothers leaving something behind and walking behind him in discipleship. There are two differences that focalize and intensify Jesus' call on the one hand and the response of the fishermen on the other. In vv. 18-20, Jesus' statement is focalized into "and he called them" (v. 22), while the statement about the fishermen leaving behind implements in v. 20 is intensified in the second section when it is said that John and James leave their father as they follow Jesus.
Jesus calls fishermen. Matthew had already mentioned that Jesus dwelt in Caphernaum by the sea at the beginning of the ministry. It would be logical then that the first fruits of his preaching would be fishermen. However, the fact that fishermen were the objects of Jesus' call may have a basis beyond mere inference. Ezekiel mentions that one of the signs of the last day will be that of people fishing where there was none before, in the Dead Sea1. Habakkuk on the other hand uses the image of fishermen as he complains to Yahweh about the Babylonians who seem unstoppable in spreading the boundaries of their empire.
(You) make men like the fish of the sea,
like the creeping things, that have no ruler over them
One takes up all of them with the hook.
He catches them in his net,
and gathers them in his dragnet. Therefore he rejoices and is glad.
Afterwards he sacrifices to his net,
and burns incense to his dragnet,
because by them his life is luxurious, and his food is good.
Will he therefore continually empty his net,
and kill the nations without mercy? (Habakkuk 1:14-17)
Both the above passages taken together may as well instruct us as to why Jesus calls fishermen: (a) because the last days have come, and (b) because the growth of the kingdom he preached will be carried out by "fishers of men." Benedict XVI explained the symbolism of the fisherman's ring he wears as Pope as a reminder of his and the Church's work. "When men who naturally live on land are fished out of the sea," he says, "it is to save them from death and restore them to life." To be fishers of men then is to fish them for the reign of God (cf. Mt. 13:47-50).
Jesus' call involved an act of "seeing" and an invitation with a promise. There is an instance in the OT where sight is associated with selection (Tagalog: paghirang). This is in 1 Samuel 16 where the verb 
ordinarily the verb for "to see" has the meaning "to choose". Jesus sees and his act of seeing becomes an invitation ("Follow me") with a promise ("I will make you fishers of men"). The invitation is respoinded to with alacrity "immediately". But the disciples' act of following is accompanied by an act of "leaving behind". In the case of the fishermen, the implements of their profession (nets, boat). In the case of "sons", their father as well. This requirement of discipleship -- the leaving behind of everything else for the sake of the kingdom of heaven -- will also be the content of Jesus' instructions to his disciples (cf. Mt. 8:18-22; 10:34-39; 16:24-28, but especially 13:44-46).
The promise "I will make you fishers of men" is about a transformation that Jesus will effect in those he calls. The grace of the ministry is a future gift for these fishers of fish. They will still have to accompany Jesus and wait for the time when he actually begins to associate them to and engage them in his work (Mt. 10:1-4). It is only after the Resurrection however when that gift is fully given for a universal -- that is, catholic -- mission (Mt. 28:16-20).
With Mt. 4:22, the story of the call of the first disciples, the first fruits of Jesus' ministry, ends. Matthew then continues the narrative with verses 23-25 as a bridge leading to the great crowd that gathers for the Sermon on the Mount. In this summary section, Matthew explains that by his preaching, healing and exorcising, Jesus begins to gather in multitudes. The liturgical selection for the Third Sunday in OT (A) breaks off after verse 23. This is so because the same verse 23 recalls Mt. 8:16-17 where a passage of the Servant Songs is cited: "He took our infirmities and bore our diseases." (Is. 53:4) Mt. 8:17 is prefaced by a summary statement on Jesus' activity:
And he was exorcising spirits with a word
and he was healing those who had diseases.
By explicitly stating that this scenario is a fulfillment of Is. 53:4, Matthew is declaring that Jesus' success in healing is due to his being the Servant of Yahweh, a designation that was implicitly contained in the voice from heaven's introduction of him at his baptism: "This is My Son, the Beloved, in whom I am well pleased."
The liturgy therefore concludes the narrative of the beginning of Jesus' ministry with an indirect reference to the Servant of Yahweh. Jesus, the one who has been set up as the light to the nations (Is. 42:6) has now begun to shine on those in darkness, associating with himself prophetic signs of the last days (cf. Ezekiel 47:10) as he inagurates the reign of God.
1. In Ezekiel 47:10, the expression "from Engeddi to Eneglaim" refers to both sides of the Dead Sea, in the same manner as "from Dan to Beersheba" refers to the whole of Northern Israel.
