Jesus took our sin upon Himself so that we might inherit His perfect obedience, His suffering, His death, His righteousness, His holiness. Jesus was treated as a sinner in His baptism, so that we might become as God's children in our Baptism. The water of the Jordan was a foul stream, filled with the pollution of our idolatry and immorality, our greed and gossip, our disobedience and rudeness, our prejudice and pride and indifference to others, our murders and thefts, our easy tolerance for sin and our intolerance of others, our selfishness and self-centeredness. Jesus took a bath in our filthy bath water. He soaked up our sin and its death into His own flesh so that He might put it to death once and for all on the cross.
Baptism also means rebirth and renewal.
It gives new birth and new life in the life of Jesus. Christ appeared to bring an end to all things and to make all things new. He came to put to death the old and sinful in His death, and to raise to life the new in His resurrection. This is why the Spirit chose the visible form of a dove. Just as the Holy Spirit once hovered over the waters of creation, and just as the dove flew over the waters of the Flood at the time of Noah, so the Spirit now descends as a dove on Jesus, showing that the new creation had come with the coming of Christ in the flesh.