This is an unusual and unexpected section of John’s Gospel. After one of the most important passages in the entire Gospel of John, the writer uses the words of John the Baptist as a summary of chapter 3.
But there may be a theological motivation to the inclusion of yet another testimony from John the Baptist at this point in the Gospel. The writer has already made it clear the Baptist was not the messiah. But after Jesus has revealed himself with two signs, there are apparently some disciples of John who have not joined Jesus. They remain loyal to John and they continue to preach the coming of the messiah and offer a baptism of repentance to prepare for the coming Kingdom of God. Jesus is now attracting disciples and followers, and he his gathering more followers than John is. His disciples naturally wonder about this and are jealous of Jesus’s success. They more or less complain to John that the “other guy” is baptizing people too. Maybe they think Jesus is working on “our turf.” John the Baptist must therefore clarify his role once again now that Jesus has begun his ministry.
It may have appeared that Jesus was in competition with John, prompting someone to ask about purification (vs 25). The issue may have been “which baptism is superior, Jesus’s or John’s?” It is disciples of John who had not started to follow Jesus who ask this question. There were at least some disciples who remained with John until he was executed and there was a group of believers who accepted only the teaching of John and not Jesus’s teaching.
In Acts 18, Apollos only knew the Baptism of John and in Acts 19 Paul encounters a group of disciples of John the Baptist who had never heard there was a Holy Spirit. They could have heard John’s preaching after Jesus was baptized and for some reason never heard the teaching of Jesus or the preaching of the Apostles after Pentecost. They returned to Ephesus without hearing the preaching of the apostles. As remarkable as it is, they were faithful to the teaching of John the Baptist some 25 years later!
It is possible this community still existed in Ephesus when John wrote his Gospel, even though another 25 years have passed. This seems possible to me. The writer of the Gospel of John could be in dialogue with both traditional Jews in the synagogue and the remnants of the Baptist’s movement.
By the end of John 3, the writer introduced Nicodemus as a well-meaning Jewish teacher who did not fully believe the message of Jesus. Hie may have thought becoming a disciple of Jesus entailed an admission he was a sinner in need of salvation. One must be “born again” to be a disciple of Jesus and Nicodemus may not have realized at this point that he was in need of repentance and regeneration.
In a very similar way, this final section of John 3 concerns the skepticism of the disciples of John the Baptist. They wonder if Jesus is superior to their own teacher. Why follow Jesus when it is possible John’s preaching and baptism are actually superior? The Baptist himself says Jesus is superior because he has come from God and is a direct witness to the will of the Father.
Both groups may have been represented in John’s community in the late first century in Ephesus. It is likely there were still Jews who appreciated some of Jesus’s teaching but could not accept his call to repent and surrender to Jesus as the ultimate representative of God.
Jesus is not asking for kind appreciation nor is he in competition with anyone’s “ministry,” still is still looking for disciples to surrender to him and follow him.