All three annual cycles for the Sunday readings reserve the Third Sunday of Advent for a narration about John the Baptist. “Our principal sources of information about John the Baptist are (1) references to his birth in the first chapter of Luke, (2) references to his preaching and his martyrdom in the Gospels, with a few references in Acts, and (3) references in Josephus to his preaching and martyrdom, references which are consistent with the New Testament ones, but sufficiently different in the details to make direct borrowing unlikely.
According to the Jewish historian Josephus (who wrote after 70 AD), John the Baptist was a Jewish preacher in the time of Pontius Pilate (AD 26-36). He called the people to repentance and to a renewal of their covenant relation with God. He was imprisoned and eventually put to death by Herod Antipas (son of Herod the Great, who was king when Jesus was born) for denouncing Herod’s marriage to Herodias, the wife of his still-living brother Philip. In order to marry Herodias, Herod divorced his first wife, the daughter of King Aretas of Damascus, who subsequently made war on Herod, a war which, Josephus tells us, was regarded by devout Jews as a punishment for Herod’s murder of the prophet John.
In the Book of Acts, we find sermons about Jesus which mention His Baptism by John as the beginning of His public ministry (see Acts 10:37; 11:16; 13:24). We also find accounts (see Acts 18:24; 19:3) of devout men in Greece who had received the baptism of John, and who gladly received the full message of the Gospel of Christ when it was told them.
Luke begins his Gospel by describing an aged, devout, childless couple, the priest Zechariah and his wife Elizabeth. As Zechariah is serving in the Temple, he sees the angel Gabriel, who tells him that he and his wife will have a son who will be a great prophet, and will go before the Lord “like Elijah.” (The Jewish tradition had been that Elijah would herald the coming of the Messiah = Christ = Anointed = Chosen of God.) Zechariah went home, and his wife conceived. About six months later, Gabriel appeared to the Virgin Mary, a kinswoman of Elizabeth, and told her that she was about to bear a son who would be called Son of the Most High, a king whose kingdom would never end. Thus Elizabeth gave birth to John, and Mary gave birth six months later to Jesus.
After describing the birth of John, Luke says that he grew, and “was in the wilderness until the day of his showing to Israel.” The people of the Qumran settlement, which produced the Dead Sea Scrolls, sometime use the term “living in the wilderness” to refer to residing in their community at Qumran near the Dead Sea. Accordingly, it has been suggested that John spent some of his early years being educated at Qumran.
All of the gospels tell us that John preached and baptized beside the Jordan river, in the wilderness of Judea. He called on his hearers to repent of their sins, be baptized, amend their lives, and prepare for the coming of the Kingship of God. He spoke of one greater follow Jesus. Some of John’s followers resented this, but he told them: “This is as it should be. My mission is to proclaim the Christ. The groomsman, the bridegroom’s friend, who makes the wedding arrangements for the bridegroom, is not jealous of the bridegroom. No more am I of Jesus. He must increase, and I must decrease.” (John 3:22-30).
John continued to preach, reproving sin and calling on everyone to repent. King Herod Antipas had divorced his wife and taken Herodias, the wife of his (still living) brother Philip. John rebuked him for this, and Herod, under pressure from Herodias, had John arrested, and eventually beheaded.” (source: “Birth of John the Baptist” by James Kiefer from the website Saints and Commemorations of the Episcopal Church).