Each Gospel has its own way of presenting what each thought necessary to know about Jesus: that John the Baptist was a witness to the identity of Jesus. John's witness was extremely powerful, because John was so greatly admired by everyone. Though he did not become a follower of Jesus, his testimony is still reliable; indeed, so valuable is John that the story of salvation through Jesus cannot begin without him.
Among the Gospels, John's is the only one to record the Baptist as calling Jesus 'the Lamb of God'. Though there are a number of suggestions about the significance of 'the lamb', probably the connection between the substance of the Book of Revelation and John's Gospel means to highlight Jesus as 'that lamb who was slain, but is now standing in the presence of God'. In this way, the Gospel early on points to the inevitable death and equally inevitable resurrection it will soon describe.
Gone from John's presentation are the words of the Father, "This is my Son, my beloved, with whom I am well pleased". Indeed, gone is a reference to the Father. What John concentrates on is the coming upon Jesus of the Holy Spirit, and on its remaining with him. In this way, the Gospel (as do others) suggests that the powers Jesus exhibits in the next pages of the Gospel are influenced by the Spirit of God. John is most eager in this context to make sure his readers know that the one on whom the Spirit comes is the one who will baptize in the Spirit, and not just with water (as does John). John is no Jesus; moreover, the baptism, with which the Gospel readers have all shared, has as its source this Jesus, this Lamb, who shares the Spirit of God with the baptized. One is not simply enveloped in water now, as with John's baptism, but one is wrapped in the Spirit, who cleanses and enlivens in the way that love does these things - and water does not.
As Chapter 5, 32-40 shows, there are many witnesses to Jesus, if one understands them and him rightly. But John does his essential part in this witnessing, a task which Jesus enjoined on his disciples (Acts 1,8); they are to testify, in whatever way they can, that Jesus is the one who gives the Spirit of God to be one with the baptized. Slowly, the Gospels and Acts begin to suggest that every baptized is potentially a witness to Jesus, in accord with his or her way of life.