That is really the enormous picture of who Jesus is. That was the first answer ever given to the question of Jesus' identity. He was asking people to commit themselves to change. But he knew that this was not the final answer to sin. People don't just decide to change and stay changed forever.
People backslide into sin again and again, returning to the ways that harm themselves and others.
Changes in behavior are too hard to keep up on your own. John knew there must be more coming in God's plan to free the world from it's great brokeness, from the power of sin. And when he gazed upon Jesus, John saw the rest of that plan that God had set in motion for the great chasm of sin between humanity and God to be removed forever. John gazed upon Jesus and saw the Spirit of God resting there, peacefully, upon him. It would not go away, this Spirit.