His birth took place six months before that of Jesus, and according to the Gospel account was expected by prophecy (Matt. 3:3; Isa. 40:3; Mal. 3:1) and foretold by an angel. Zacharias lost his power of speech because of his unbelief over the birth of his son, and had it restored on the occasion of John's circumcision(Luke 1:64).
As an adult John started preaching in public, and people from "every quarter" were attracted to his message. The essence of his preaching was the necessity of repentance and turning away from selfish pursuits. He denounced the
Sadducees and Pharisees as a "generation of vipers," and warned them not to assume their heritage gave them special privilege (Luke 3:8). He warned tax collectors and soldiers against extortion and plunder. His doctrine and manner of life stirred interest, bringing people from all parts to see him on the banks of the Jordan River. There he baptized thousands unto repentance (see AEnon).
The fame of John reached the ears of Jesus in Nazareth(Matt. 3:5), and he came from Galilee to Jordan to be baptized by John, on the special ground that it became him to "fulfil all righteousness" (3:15). John's special office ceased with the baptism of Jesus, who must now "increase" as the King come to his kingdom. He continued, however, for a while to bear testimony to the Messiahship of Jesus. He pointed him out to his disciples, saying, "Behold the Lamb of God." His public ministry was suddenly (after about six months probably) brought to a close by his being cast into prison by Herod, whom he had reproved for the sin of having taken to himself the wife of his
brother Philip (Luke 3:19). He was shut up in the castle of Machaerus, a fortress on the southern extremity of Peraea, 9 miles east of the Dead Sea, and here he was beheaded at the instigation of Herodias; later tradition also implicates Salomé. His disciples, having consigned the headless body to the grave, went and told Jesus all that had occurred (Matt. 14:3-12). John's death occurred apparently just before the third Passover of Jesus' ministry.
Jesus himself testified regarding John that he was a "burning and a shining light" (John 5:35). The Eastern Orthodox believe that John was the last of the Old Testament prophets, thus serving as a bridge figure between that period of revelation and Jesus. They also embrace a tradition that, following his death, John descended into Hell and there once more preached that Jesus the Messiah was coming.
In Gnosticism, John the Baptist was a "personification" of the Old Testament prophet Elijah. According to Gnostic theology, John the Baptist was a prophet from the Old Testament who did not know the True God (the God of the New Testament, as opposed to the God of the Old Testament — see the article on Gnosticism for details), and thus had to be reincarnated. As predicted by the Old Testament prophet Malachi, Elijah must "come first" to herald the coming of Jesus Christ.
John the Baptist indeed does "come first" before Jesus in all four Gospels, however, in the canonical Christian view, John the Baptist is not Elijah. Indeed he directly denied being Elijah according to the Gospel of John 1:20: when the Jews ask him if he is the Messiah, he empathically denies this. Then they ask him if he is Elijah, to which he answers "I am not.". According to canon, John does fulfill the prophesy of Malachi, but is not the reincarnated prophet Elijah.