When thinking of the public ministry of Jesus, I usually think of it as beginning with the Sermon on the Mount. This would not be accurate.
Immediately following his baptism and Satan’s temptation in the wilderness, Jesus went to Galilee and dwelled in Capernaum among the Gentiles. Of this period, Matthew says it was the fulfillment of Isaiah 9:1-2. The public life and ministry of Jesus began among the Gentiles, “the people who sat in darkness… and in the region and shadow of death” (vv 15-16). Matthew goes to record that “from that time Jesus began to preach and say, Repent, for the kingdom of heaven is at hand” (v 17).
In verses 18-22 we see the beginning of Jesus’ call for specific disciples, specifically Peter and Andrew followed by James and John. We would be foolish to think these four men simply left their jobs and families (James and John worked with/for their father Zebedee) to follow a random stranger.
First, we know that all of the Twelve were first disciples of John the Baptizer before becoming disciples of Jesus, as this was a requisite for whomever would replace Judas in the tenor so days between Jesus’ ascension and Pentecost (Acts 1:15-22).
Second, John made no secret that the Messiah would come in those days. John’s message was more than one of simple repentance. He was working to open the eyes of the people to this One who would come after. So, the Twelve were keenly aware of His coming when they heard and believed the preaching of John.
Third, John made known the identity of the Messiah when Jesus was in his presence. In John 1:29, John saw Jesus walking toward him and declared, “Behold! The Lamb of God who takes away the sins of the world! This is He of whom I said, ‘After me comes a Man who is preferred before me… and I have seen and testified that this is the Son of God.’”
Matthew 4:23-25 mentions the itinerant work of Jesus leading to the gathering of multitudes following Him from place to place. The Sermon on the Mount is simply the first recorded sermon of the Lord.