I like the way Dr. H. A. Ironside explained this: He said that we are like paupers who have accumulated so many debts that we cannot pay them. These are our sins. These tremendous claims are made against us, and we cannot possibly meet them. But when Jesus came, he took all these mortgages and notes and agreements we could not meet and endorsed them with his own name, thereby saying that he intended to pay them, he would meet them. This is what his baptism signifies, and is why Jesus said to John the Baptist, "...thus it is fitting for us to fulfill all righteousness," (Matthew 3:15b RSV). He declared his intention to meet the righteous demands of God by himself undertaking to pay the debts of men. So the baptism was clearly an act of identification.