Verses 19-28 explain who John is. In fact, verse 19 says, And this is the testimony of John. The Jews (an expression used 68 times in John and often referred to those opposed to Christ) asked “Who are you?” and John emphatically stated that he is not the Christ (“I am not the Christ.”). John will not make any messianic claim for himself. But John’s denial is considered a confession as v. 20 states, He confessed [ὡμολόγησεν], and did not deny, but confessed [ὡμολόγησεν].
When asked, “Are you Elijah?” John says “I am not.” [Οὐκ εἰμί] Elijah was expected as Mal. 4:5 indicated; remember, he never died (1K. 2:11). John’s clothing (Mk. 1:6) strongly suggested a link to Elijah (1K. 1:8) but he denies that he is. Though Jesus claimed that John in fact came as Elijah (Mk. 9:13), John was not willing to claim that for himself. The question, “Are you the prophet?” suggests they were asking John if he was THE prophet spoken of in Deut. 18:15, 18 (cf. Jn. 6:14; 7:37). To this question, John says No. Jesus is that prophet (cf. Acts 3:22; 7:37) but John is merely a prophet (Mt. 11:11-14; Jn. 10:40-41).
The frustrated “members of the deputation” need to know who John was. We need to give an answer to those who sent us indicates John was someone significant. So John quotes Isaiah 40:3, “I am the voice of one crying out in the wilderness, ‘Make straight the way of the Lord,’ as the prophet Isaiah said.” The Jews returning from Babylon to Jerusalem were to spiritually change in their return. John on the other hand is calling the Jews to prepare for the coming Messiah.
“Then why are you baptizing, if you are neither the Christ, nor Elijah, nor the Prophet? That is, John had to have a theological reason for why he was baptizing?
Baptism was not unknown among the Jews. It was self-administered by Gentiles who became Jewish proselytes (and by members of the Qumran sect for ritual cleansing). But John himself was administering the baptism and those he baptized were already Jews.
The Pharisees could see why the Gentiles need baptism and why someone like the Messiah might baptize but John is neither the Christ nor baptizing Gentiles. This did not make sense. But John’s answer is important. His baptism is only with water, I baptize with water. Why this point? “This should not be taken as indicating that he does not regard his baptism as important. He does. He does not depreciate it. But his baptism is not an end in itself. Its purpose is to point people to Christ (v. 31).” This is a point we must never forget about baptism. John’s baptism as well as anyone’s baptism after are never an end in themselves — they must point to Christ!
John says, but among you stands one you do not know, even he who comes after me, the strap of whose sandal I am not worthy to untie. This means that his baptism compared to what is coming after is really nothing — a far more significant “baptism” will come (“He will baptize you with the Holy Spirit and fire.” Mt. 3:11). Let us also observe John’s humility before the Lord. He has not even met him yet and we see his understanding and humility before the coming Messiah. Ryle’s comments are instructive and necessary:
Yet here in this passage we see this eminent saint lowly, self-abased, and full of humility. He puts away from himself the honor which the Jews from Jerusalem were ready to pay him. He declines all flattering titles. He speaks of himself as nothing more than the “voice of one crying in the wilderness,” and as one who “baptized with water.” He proclaims loudly that there is One standing among the Jews far greater than himself, One whose shoe-latchet he is not worthy to unloose. He claims honor not for himself but for Christ. To exalt Christ was his mission, and to that mission he steadfastly adheres.
The greatest saints of God in every age of the Church have always been men of John the Baptist’s spirit. In gifts, and knowledge, and general character they have often differed widely. But in one respect they have always been alike–they have been “clothed with humility.” (1 Pet. 5:5.) They have not sought their own honor. They have thought little of themselves. They have been ever willing to decrease if Christ might only increase, to be nothing if Christ might be all. And here has been the secret of the honor God has put upon them. “He that humbles himself shall be exalted.” (Luke 14:11.)
If we profess to have any real Christianity, let us strive to be of John the Baptist’s spirit. Let us study HUMILITY. This is the grace with which all must begin, who would be saved. We have no true religion about us, until we cast away our high thoughts, and feel ourselves sinners. This is the grace which all saints may follow after, and which none have any excuse for neglecting. All God’s children have not gifts, or money, or time to work, or a wide sphere of usefulness; but all may be humble. This is the grace, above all, which will appear most beautiful in our latter end. Never shall we feel the need of humility so deeply, as when we lie on our deathbeds, and stand before the judgment-seat of Christ. Our whole lives will then appear a long catalogue of imperfections, ourselves nothing, and Christ all.
 One writer put it this way, “John the Baptist himself still saw Elijah as a messianic figure and so shrank from identification with him. Implicit in his denial is the assumption that the One coming after him is Elijah, as well as the Prophet and the Messiah. “ J. Ramsey Michaels, John (NIBC 4; Accordance electronic ed. 18 vols.; Peabody: Hendrickson Publishers, 1989), 31.
 Colin G. Kruse, John: An Introduction and Commentary (TNTC 4; IVP/Accordance electronic ed. Downers Grove: InterVarsity Press, 2003), 79.
 Leon Morris, The Gospel of John (NICNT; Accordance electronic ed. Grand Rapids: Eerdmans, 1995), 123-124.