Jesus Proclaims the kingdom. (12-17)
v.12 Jesus begins his public ministry in Galilee having left Judea when he learned that John the Baptist had been arrested by Herod Antipas (see also 14:1-12). Jesus may have remained for some time in Judea after his baptism (e.g. Jn 1:26-4:3). His withdrawal to Galilee may show us not to look for persecution but rather to withdraw from its sphere of influence.
v.13-16 Jesus moves to Capernaum by the sea (of Galilee). This fulfilled the Prophecy in Is 9:1-2, the region of Galilee (the territory of Zebulun and Naphtali) would see a great light (i.e., the gospel). Galilee, with a large gentile population, points to the extension of the gospel to all nations (Mt 28:19).
v.17 Jesus began to preach: “Repent, for the kingdom of heaven is at hand.” (cf. John the Baptist’s preaching in 3:1-2). Carson sees Jesus as a new Joshua, leading his people into the promised land (EBC). John brought to a close the old covenant and Jesus was the beginning of the new covenant.
Jesus calls disciples to follow him. (18-22)
v.18-20 Jesus calls the brothers Peter and Andrew to follow him (i.e., leave their occupation as fishermen and accompany him full time). Peter, along with John and James, will become part of Jesus’ inner circle. Jesus says he will make them fishers of men (evangelists). They immediately drop their nets and follow him.
v.21-22 Farther on Jesus saw the sons of Zebedee, James and John, in a boat mending nets with their father. He called them and they at once left their father and followed Jesus. When Jesus calls you to discipleship their is no excuse for delay. A call from the Lord involves commitment and cost, they left the family business and committed to following Jesus in his messianic mission.
Great crowds follow Jesus. (23-25)
v.23 Jesus proclaimed the gospel (good news, NIV) of the kingdom and healed the sick throughout all Galilee. He preached in the synagogues on Saturdays and to (open air) crowds during the week. Galilee had a large population in about 240 villages, each with 15,000 or more persons, according to Josephus (Life 235, War 3.iii.2). By divine power he healed their bodies and by the medicine of his preaching he healed their souls.
v.24 His fame spread throughout all of Syria (a region north of Galilee) and they brought him all who were sick with various diseases or oppressed by demons and he healed them.
v.25 Great crowds followed him from Galilee, Decapolis, Jerusalem, Judea, and beyond the Jordon river. It is likely that Jesus taught some of the same material again and again and faced many of the same problems and diseases again and again.