In these verses it never said it was the baptizing that took away the sins of the world, but the confession and repentance that changes the inward man. Baptism is an outward indication of what has taken place inward when he has already repented and believes that Jesus Christ is the Son of God. So it is a work that becomes a public testimony of the cleansing that has taken place in the soul. It is a type of Jesus' death and resurrection. Our old nature or sin is dead and we have become new and clean.
Baptism is a command from Jesus.
Jesus taught us the way to follow Him, in the name of the Father, the Son and Holy Ghost. In verse 20 again, teaching to observe all things I have commanded you. So you are commanded to be baptized as a testimony to the cleansing power of our Lord to show others His mighty work in your life. He changed you from the inside out. There are all kinds of arguments about what to use.