Though we know him as John the Baptist, very little notice is given in this Gospel to his baptising ministry; rather it is his role as a witness who has come to testify about Jesus that is emphasized. That was how John was introduced to us in verses 7 and 15, and that is now unpacked and explained. The passage recounts the events of two consecutive days: day one is full of questions; on day two we get the answers.
Like the apostle Paul, who could say “We do not preach ourselves, but Jesus Christ as Lord”, so John is at pains to point away from himself. His ministry clearly attracted considerable interest, prompting a delegation from Jerusalem to come to investigate. It seems they wondered whether John was making any claim to be Messiah, but John denies that emphatically. They reach around for other Old Testament categories to put him in – first Elijah, whom Malachi had prophesied God would send before “the great and terrible day of the LORD” (Mal.4:5), then the Prophet, whom Moses had promised (Deut.18:15-18). John denies both, wanting only to point away from himself. The Old Testament scripture he prefers to point them to is Isaiah 40, identifying himself as only “the voice”, which takes the focus of attention away from the identity of the speaker and puts it rather on the message: he is the one who announces the imminent arrival of the LORD Himself. I think it is worth looking at Is.40:3 in context to understand more what John was witnessing to: the comfort of sins finally dealt with, the revelation of God’s glory to all mankind, the coming of God to his people.
Some maybe be confused by John’s denial that he is Elijah, given that elsewhere Jesus says he was Elijah (Matt.11:14; cf Luke 1:17). It may be that John was not aware of his own significance (he certainly has no wish to draw any attention to it), but it may be simply that there was a sense in which he was Elijah (in terms of his role) and a sense in which he wasn’t (ie he wasn’t literally Elijah, reappearing on earth, having not tasted death in 1 Kgs 2 when he was taken up into heaven).
John doesn’t really answer the Pharisees’ question as to why he baptises, rather he points again away from himself to someone far greater, implicitly suggesting they should be wondering, not about his ministry, but about the ministry of the one he was testifying about (which we will learn in verse 33 will be to baptise not with water but with the Holy Spirit.)
John’s whole purpose was to prepare people for the arrival of Jesus (hence baptism), and to point him out, which is what he does here, clearly and unequivocally. The manner in which John identifies Jesus, though of course familiar to us, is surprising. The previous dialogue with the delegation from Jerusalem has set us up to make sense of Jesus in Old Testament categories, but they (and we) were not expecting “the Lamb of God”. I suspect John (the Gospel writer that is) is alluding to the Servant of Isaiah 53, who is likened to a lamb and who would bear the sin of many. (Commentators debate what John the Baptist meant, and it may well be that he was thinking rather of a messianic figure who take away the sin of the world by bringing judgment, but the more important thing is what the writer of this Gospel is wanting to understand through John’s testimony, for like Caiaphas in 11:50 the Baptist may have spoken more truly that he understood).
John then explains how he came to recognise who Jesus is – ie not just as his cousin, but as the one whose coming he was to herald. Jesus is the one on whom the Spirit rests, and who will himself bring the long-promised blessing of the Spirit to God’s people. Again our ears should be attuned to pick up the Old Testament background – eg Isaiah 11, 61, and Ezekiel 36 and Joel 2. Jesus is the Son of God (here perhaps chiefly a messianic title, cf Psalm 2), the Christ (or Anointed One), who has come to usher in the fulfilment of all God’s promises. In particular John has pointed to the fact that Jesus will (i) take away sin, and (ii) pour out the Spirit.