After a period of desert solitude, he attained notice as a prophet in the region of the lower Jordan Valley. He had a circle of disciples, and Jesus was among the recipients of his rite of baptism.

Sources of information about John. The primary sources for information about John's life and activity are the four Gospels (Matthew, Mark, Luke, and John), the Acts of the Apostles (Acts of the Apostles, The), and the Jewish historian Josephus' Antiquities of the Jews (Antiquities of the Jews, The). In using these works for historical reconstruction, allowances must be made for the known tendencies of each writer. All four Gospels recognize in John the start of the Christian Era, and each in its own way tries to reconcile John's precedence in time and Jesus' acceptance of his message and of a baptism of repentance from his hands (elements suggesting subordination to John), with the author's belief in Jesus as Messiah and Son of God. The Gospel According to Mark (Mark, The Gospel According to) presents Jesus as the hidden Messiah, known only to a narrow circle, and John as the prophet Elijah returned and as the one who had to “come first to restore all things” but who also remained hidden and suffered death with little acknowledgment of his true status (Mark 9). An early collection of sayings of Jesus, known to biblical scholars simply as Q, similarly represents the Baptist as the herald of the Coming One and of the imminent Kingdom, yet tries to stress his preparatory character, and so his subordination to Jesus. Matthew and Luke develop these two sources. The Gospel According to Matthew (Matthew, Gospel According to) emphatically identifies John as the returning Elijah, herald of the Kingdom of God (Matt. 3). For Matthew, John's death, like that of Jesus, illustrates the old Israel's hostility to God's offer of salvation. Luke, in his Gospel and in the Acts of the Apostles, neglects the identification with Elijah but describes John as Jesus' forerunner and as inaugurator of the time of fulfillment of prophecy. Luke's account of the infancy of John and of Jesus does not necessarily derive from an underlying document composed in a “Baptist” sect, although it uses material perhaps transmitted by former disciples of the Baptist. It depicts the birth of Jesus and John in two parallel series of scenes, each with an angelic annunciation, conception, marvellous birth, circumcision, hymns greeting the children and predicting their destiny, and infancy. Even in his mother's womb John recognizes Jesus—also still in his mother's womb—as his Lord. The Gospel According to John (John, Gospel According to) reduces the Baptist from an Elijah to a model Christian preacher, a mere voice; it omits any description of Jesus' baptism. Its tendency has often been labelled polemic against a continuing group of disciples of John, but it is more plausibly explained by the evangelist's desire that this ideal witness recognize the full character of the Christ as he is presented in the Gospel According to John and as a necessary consequence of the tension between the highly developed understanding of Christ in this Gospel and those details in early Christian tradition that suggested Jesus' subordination to John. The Gospels are thus primarily interested in the relations between John and Jesus.

Josephus (Josephus, Flavius) sought to present Jewish religious phenomena in Hellenistic categories and to deemphasize any political elements unfavourable to Roman imperial control.

After allowances are made for the tendencies of each of these sources, the following items about John appear relatively trustworthy. He was born somewhere in Judaea (localized at ʿEn Kerem from at least AD 530) to Zechariah, a priest of the order of Abijah, and his wife, Elizabeth, perhaps a relative of Mary, the mother of Jesus. His formative years were spent in the Judaean desert, where monastic communities, such as the Essenes (a strict Jewish sect that existed from about the 2nd century BC to the end of the 1st century AD), and individual hermits often educated the young in their own ideals.

In 27/28 or 28/29 John attained public notice, not as a priest but as a prophet. He was active in the region of the lower Jordan Valley, from “Aenon near Salim” (near modern Nablus) to a point east of Jericho. His dress of an austere camel's hair garment was the traditional garb of the prophets, and his diet of locusts and wild honey represented either strict adherence to Jewish purity laws or the ascetic conduct of a Nazirite (a Jew especially vowed to God's service). His mission was addressed to all ranks and stations of Jewish society. His message was that God's wrathful judgment on the world was imminent and that, to prepare for this judgment, the people should repent their sins, be baptized, and produce appropriate fruits of repentance. Certain problems about the meaning of John's message continue to be debated: In Matt. 3, John says, “he who is coming after me is mightier than I”; this might refer to God himself, a human messiah, or a transcendent divine being. He also says, “I baptize you with water . . . ; he will baptize you with the Holy Spirit and with fire”; this second baptism might symbolize the judgment the one coming would carry out. John's followers were characterized by penitent fasting, beyond the demands of Jewish Law, and special prayers. John's ethical call for justice and charity in Luke 3 requires righteousness from everyone in his own situation.

Although, like earlier prophets, John had an inner circle of disciples, Baptism was not an admission rite into this group. It was a rite (immersion in running water) that symbolized repentance in preparation for the coming world judgment and was to be accompanied, before and afterward, by a righteous life. It was hardly conceived as a sacrament, in the Christian sense, conveying forgiveness, or as superseding Judaism and marking off a new people, including both Jews and Gentiles, prepared for God's final Kingdom; nor is a hypothesis that it symbolized a new Israel's crossing of the Red Sea toward a new national deliverance demonstrable. Equally unprovable is that it was a rite symbolizing man's reunion with divinity and return to his heavenly home—a sacrament of salvation and rebirth. The Jewish rite of baptism of converts differs fundamentally and is not its source. There were several other baptizing groups found about the same time and place, but none of these various and little-known baptisms can be shown to have inspired John's. It may have resembled in parts the initiatory baptism of the Essenes, though their other baptisms were more concerned with maintaining their community's ritual purity. John's baptism probably symbolized not so much anticipated entrance into the Kingdom of God as an anticipatory submission to the coming world judgment, which was represented as a coming second “baptism” by the Holy Spirit in a river of fire.

The discovery of the Dead Sea Scrolls has drawn attention to the numerous parallels between John's mission and that of the Essenes, with whom John may have received some of his religious training. Both were priestly in origin, ascetic, and with intense and, in many respects, similar expectations about the end of the world. But John neither belonged to nor intended to found any organized community; he did not stress study of the Mosaic Law; and his message was more widely directed (to the poor, to sinners) than was that of the Essenes.

Jesus, who was baptized by John, saw in John the last and greatest of the prophets, the one who prepared for the coming of God's Kingdom (God, Kingdom of) (Mark 9, Matt. 11, Luke 7), and in many ways his ministry continued and developed John's. Whether John, who probably expected a divine Son of Man, recognized him in Jesus is not clear, but many of his disciples later followed Jesus.

Some time after baptizing Jesus, John was imprisoned by Herod Antipas, ruler of Galilee and central Transjordan. His crime was hardly the innocuous moral message Josephus presents, nor would his message, as found in the Gospels, have had much more immediate political bite. Herod had married (illegally, by Jewish Law) Herodias, the divorced wife of his half brother, after divorcing his first wife, the daughter of King Aretas IV of the Nabataeans, an adjacent Arab people. John's denunciation of this marriage doubtless presented Herod with the danger that his Jewish subjects would combine with his semi-Arab subjects in opposition to him. John's execution certainly preceded Aretas' victory over Herod in 35–36, a defeat popularly considered to have been divine vengeance on Herod for killing John. According to the Gospels, John's death preceded Jesus'; any greater chronological precision depends on the dates of Jesus' ministry and death. It is probable that John's followers recovered and buried his body and revered his tomb. The traditional burial site, at Sebaste (originally Samaria), near “Aenon by Salim,” is attested from 360 onward.