While John the Baptist was the last prophet of the old covenant as well as the first prophet of the New Covenant, he was more than a prophet for he was a special messenger sent to prepare the way for Christ (Matthew 11:9-13). However, even though there was no prophet greater than him, he freely admitted that he was not even worthy to take off Jesus' sandals. [Luke 3:4, Isaiah 40:1-5, Matthew 21:32, Luke 16:16, John 1:23, Mark 1:1-5, Luke 7:26-28.]
In regard to theology, he clearly taught faith in the Triune God, for he spoke of the Father as the One who sent Him to baptize (John 1:33), the Son as the One who existed before him (John 1:30), and the Holy Ghost as proceeding from both the Father and the Son. (Matthew 3:11, Mark 1:8&17, Luke 3:16, John 3:31,34, John 1:33).
He not only believed that Jesus was the Christ (the Messiah), but also believed that Jesus was God (John 3:28, John 1:36&41). In that regard, he not only called Jesus the “Son of God” — which, according to the Jewish way of thinking was the same as saying that He was Divine, John 1:34 — but also told his disciples that Jesus had come down from heaven, was above all, was mightier than he, and was the one who baptized with the Holy Spirit (John 3:31, John 1:30, Mark 1:7, John 1:27,33, Luke 3:16, Matthew 3:11). Furthermore, the passage that John quoted in reference to Jesus, “Make straight the way of the Lord” says in the Hebrew, “Prepare ye the way of Jehovah, make straight in the desert a highway for our God” (John 1:23, Isaiah 40:3).
John’s claim that he was not even worthy to take off Jesus' sandal, testifies to the fact that he regarded Jesus as God (Mark 1:7). And, because He is God, the Spirit is His without measure (John 3:34, John 3:31, Luke 7:28).
In regard to salvation, John the Baptist made it clear that Christ was the source of forgiveness, and that eternal life is ours only through faith in Him. In reference to forgiveness, he identified Christ as “the Lamb of God, which taketh away the sin of the world,” and told his disciples that Christ would “baptize” (wash) them “with the Holy Ghost” (John 1:29,36, Mark 1:8). In reference to faith, he said, “He that believeth on the Son hath everlasting life; and he that believeth not the Son shall not see life; but the wrath of God abideth on him” (John 3:36).
John was sent by God to call the nation to repentance, and John carried out that work by preaching the baptism of repentance for the remission of sins, and by baptizing all who confessed their sins (Mark 1:4-5, Luke 3:3). At the same time, he rebuked certain of the Pharisees and Sadducees who came to him, and warned them to stop thinking that they were safe just because they were descended from Abraham and to show by their actions that they were truly repentant (Matthew 3:7-11, Luke 3:7-8,16.).
John the Baptist had to deal with people who not only sought assurance of salvation in works, but also derived a false assurance of salvation from the fact that they were descendants of Abraham, or from their wealth (assuming that any wealth they had was evidence of God's favor). At the same time, he had to deal with people who had a poor understanding of repentance. For that reason, John used baptism to associate repentance with forgiveness, while giving all who confessed their sin the same message that Nathan gave to David when he repented, “The Lord also hath put away thy sin, thou shalt not die” (2Samuel 12:13, Acts 22:16).
By using baptism to convey God’s promise of forgiveness in Christ, John gave those who repented the assurance of forgiveness that they needed if they were to trust in God's mercy (Psalm 13:5). At the same time, by making it clear that Christ was the source of that mercy, John led them to trust in Him (Acts 19:4, Mark 1:8, John 1:29, Mark 1:8, Luke 3:16, Psalm 51-1-2). Because Christ was the one John held up as the source of spiritual cleansing/baptism, his baptism was more than a mere symbol (Matthew 3:11, Luke 3:16). It was the means that God used to give people His promise of forgiveness in Christ, and all who looked to Christ for forgiveness received what was promised (Galatians 3:22, 2Corinthians 1:20, Romans 5:2).
People who read Acts 19:1-7 often assume that Paul rebaptized men who had originally been baptized by John the Baptist. However, that assumption conflicts with the fact that Christ continued the work of John (John 4:1-2), and that none of the apostles [or Apollos] that had been baptized by John were ever rebaptized. Furthermore, John the Baptist referred to the Holy Ghost in his teaching, and these men had not even heard of the Holy Ghost (Mark 1:8). John also baptized “unto repentance,” not “unto John's baptism” (compare verse 3 with Matthew 3:11). Therefore, these men may have been baptized by an imitator of John the Baptist, rather than by John himself. For, not all of John’s followers accepted Christ, and one group of them still exists in the Middle East, where they are known as Mandeans.
The message of John is the message of the Christian church, the baptism of John is the baptism of the Christian Church, and the theology of John is the theology of the Christian church. John began the work of baptism, Jesus and His disciples carried on that work, and the call to baptism given by Peter on the day of Pentecost is essentially the same as that given by John. [compare Acts 2:38 with Luke 3:3, see John 3:22-23, John 4:1-2.] Instead of teaching his disciples to seek righteousness by the law, John taught them to look to Christ for forgiveness. He made it clear that works were the fruit of salvation, not what saves us, and he taught those who repented to do that which is honest, good and right apart from the law (Luke 3:10-14, Matthew 3:8).
Bibliography: Matthew 3:1-17, Mark 1:1-11, Luke 3:1-23, John 1:15-40, John 3:5,22-26, Matthew 11:7-14, Luke 7:19-35, Luke 7:20, Luke 9:9, Mark 6:14, Matthew 17:13, Isaiah 40:1-3, Matthew 4:2, Mark 1:14, Luke 16:16, Mark 6:20, John 4:1-3, John 5:33-36, John 10:41, Acts 1:5, Acts 13:24-25, Acts 19:1-7, Matthew 21:32. Only those who have the theology of John have a right to the name “Baptist.”