After the repentance (change of mind, away from the evil to GOD) the baptism is the public testimony in the visible and invisible world. This happens from the free will of a person who has understood and accepted CHRIST. The baptized one wants to leave the old behind itself and to arrange now again with its personal redeemer the life further. The old man dies and with him everything he has done, the sins are forgiven, Satan has no more claim on this human soul. On the contrary he fears it now, because now the son or the daughter of God is equipped with DIVINE authority to use this for the liberation of the whole creation which he still controls.