These notes primarily taken from Baker's Evangelical Dictionary of Biblical Theology:

Apart from Jesus Christ, John the Baptist is probably the most theologically significant figure in the Gospels. He was not a writer or traveler, yet I repeat---apart from Jesus Christ, John the Baptist is probably the most theologically significant figure in the Gospels. As was the case with Jesus, his birth was meticulously recorded (Luke 1:5-25). His entrance into the world was marked by angelic proclamation and divine intervention (Luke 1:57-80). John's birth not only parallels that of Jesus's, but echoes the momentous occasion of the birth of Isaac to Abraham and Sarah (Gen 17:15-22 ; 21:1-7). John is clearly a pivotal figure in the salvation history of God.

• John the Baptist (also called John the Baptizer or Yahya the Baptizer) is viewed as a prophet by at least three religions: Christianity, Islam, and Mandaeanism (stresses salvation of the soul through esoteric knowledge (gnosis) of its divine origin).
• Although his formative years were lived in obscurity in the desert (Luke 1:80), his public ministry ended nearly four hundred years of prophetic silence. John was that "voice crying in the wilderness" preparing the way for the coming Messiah (Isaiah 40:3; Matthew 3:3; Mark 1:2-3; Luke 3:3-6). In this sense, his message and ministry marked the culmination of the law and the prophets, but heralded the inbreaking of the kingdom of God (Matt 11:12; Luke 16:16). So John was truly a transitional figure, forming the link between the Old and New Testaments. He spans the ages with one foot firmly planted in the Old Testament and the other squarely placed in the New.
• The central theme of his ministry was, "Repent, for the kingdom of heaven is near" (Matt 3:2).He was called "The Baptist" because his practice was to baptize those who responded to the message he proclaimed and sincerely repented of their sins (Matthew 3:1; Mark 6:14; Luke 7:20).
• John was an end-times prophet. He conducted his ministry with an eschatological authority that demanded immediate action. He taught that judgment is at hand. The axe is laid to the roots and God will thoroughly purge his threshing floor (Matthew 3:10-12; Luke 3:9; Luke 3:17). And the authenticity of repentance was evidenced in very practical terms: share with those in need, eliminate graft, and prohibit extortion (Luke 3:11-14).
• John's lifestyle was as austere as his message. He was an ascetic living in the wilderness, clothed in camel hair and subsisting on locusts and wild honey (Matthew 3:4; Mark 1:6). Unlike Jesus, he expected people to come to him, rather than he going to them (Matthew 3:5).
• John was no "crowd pleaser." He willingly confronted the hypocrisy of the religious establishment (Matthew 3:7; Luke 3:7). He did not hesitate to expose the immorality of Herod and chose to die a martyr's death rather than compromise his convictions (Matthew 14:3-12; Mark 6:17-29).
• All of these characteristics portray John as a fiery prophet proclaiming the apocalyptic message of God. Indeed, Luke says that John came "in the spirit and power of Elijah" (Luke 1:17). He goes on to allude to Malachi 4:5, which states that Elijah will return "before that great and dreadful day of the Lord." In fact, some contemporaries of John inquired if he were Elijah (John 1:21).
• The belief that Elijah would return and prepare the way of the Lord can be traced to Malachi 3:1 and 4:5. Such belief is also found in the extra-biblical accounts of Sirah 48:10 and 2 Esdras 6:2f. The Gospels also indicate that many believed that Elijah would come first, and then the Christ (Matthew 11:14; 17:10; Mark 6:15; 9:11; Luke 9:8).
• John flatly denied that he was Elijah reincarnated (John 1:21; John 1:25). Nevertheless Jesus affirmed that Elijah must come first and that he had come in the person of John the Baptist (Matthew 17:11-13; Mark 9:12-13). John fulfilled Malachi's prophecy in a spiritual sense, rather than in a literal way.
• In this way, Jesus acknowledges the central role that John played in God's plan of salvation. He was the greatest born among women because he had the privilege of pointing to the Lamb of God (John 1:29-34).Yet as the last great prophet of the pre-Christian era, he was the least in the kingdom of God (Matthew 11:11; Luke 7:28).
• John fully accepted his subordinate role to Christ. He denied that he was the Christ and repeatedly emphasized that he was simply a witness to the Light (John 1:19-23; cf. also John 1:6-9; John 3:27-30). John stated that Jesus was greater than he, and that Jesus had a more powerful ministry and baptism (Mark 1:7-8; Luke 3:16; John 1:26-27). He did not want to baptize Jesus, but rather desired to be baptized by Jesus (Matthew 3:13-14).John allowed his disciples to leave his own leadership and follow after Jesus (John 1:35-39).
• But for all of his greatness, John was merely human. In this sense, he too joined in the popular speculations about the identity of Christ. It may be that John's vision of the Messiah varied so much from what he heard and saw in Jesus, that he came to question if Jesus were really the Christ (Matthew 11:1-2; Luke 7:18). The fact that Jesus was not an ascetic, and that he actively sought the fellowship of publicans and sinners may have been an offense to John and his disciples (Matthew 9:9-17; Matthew 11:18-19; Luke 7:33-34).Jesus may have rebuked John in this regard when he said, "Blessed is the man who does not fall away on account of me." (Matthew 11:6; Luke 7:23).
• The topographical setting of John's ministry may be of theological significance. The desert setting may underscore the stark nature of John's message or may be symbolic of Israel's struggle in the desert.
• Finally, even though John was merely a witness serving as a transitional figure, the impact of his life and ministry should not be underestimated. During his lifetime he had a following of disciples who shared common practices such as fasting and prayers (Matthew 9:14; John 1:35-37; 4:1-2). John's disciples survived his death and spread throughout the Mediterranean world. Apollos was from Alexandria in North Africa and at one point knew only of the baptism of John (Acts 18:24-25). Similarly, upon arriving in Ephesus, Paul encountered about a dozen disciples of John. They too had only experienced the baptism of John. (Acts 19:1-7 . These instances indicate that the Baptist's movement may have had more influence than what we are able to glean from the New Testament.
• As we close our story about John the Baptist. I want you to think how fast this last year went. Or the past two years. The tragic death of John the Baptist and the death of our Lord Jesus were not that far apart. They were cousins---what did Jesus think about his cousin John?
• Jesus saw John's faithfulness and courage to stand for the righteousness of God.
• How does Jesus see our faithfulness?