The general subject of the passage seems to be authority and power.
John himself is the initial focus of the conversation at first, but he redirects the focus to the one who was coming who would be greater in power and authority. That person was a person God had told him would come and who turned out to be Jesus.
The first point John makes is that there is importance in water baptism as he has been sent by God to do. He then describes a more important baptism of the Holy Spirit that the one who was coming after him was going to do.
John describes seeing Jesus and feeling that he was “the Lamb of God who takes away the sin of the world.” For those listening to John at the time, this description would bring up thoughts of the scapegoats, the yearly sacrifices done on the Day of Atonement by the High Priest for the whole nation. John is describing Jesus as that sacrifice for the whole planet. (info about Scapegoating)
Then John progresses to the idea that Jesus is not only a person greater than him with a greater baptism, but that he is an eternal being. John says: “A man who comes after me was before me because he existed before I did” and goes on to show how this was both foretold by God to him and verified publically when Jesus was baptized by John through the Holy Spirit’s coming to rest on Jesus.
John completes the thought with the clear statement that Jesus is the “Son of God”
So John was given authority to baptize and was sent into his ministry By God, But God had told him that one greater was to come later to baptize with the Holy Spirit and that God would show him who this person was. This person, who turned out to be Jesus was to actually be the Son of God revealed.
The passage is making clear who Jesus was (is) and how that he brings us a deep relationship with God through the Holy Spirit. A relationship with God different than anything that people had before Jesus came. It is also making the bigger point that Jesus is the Son of God.
This understanding and belief in this is foundational to calling yourself a Christian.
The message for me personally through this passage seems to be to focus on what Jesus is doing beyond what I am doing and even beyond what He is doing through me. Remember there is always a bigger picture in God’s eyes.