Q. Where did John the Baptist get the idea of water baptism? I see it mentioned in the O.T. regarding leprosy, but don’t see in the O.T. water baptism for repentance. Also, why did Jesus agree to John’s baptism? Just to set an example? But that would make it seem that He too was in need of repentance, wouldn’t it? Which of course is not the case.
Biblical scholars generally agree that the roots of water baptism are in the requirement in the Law of Moses that people bathe with water to return to a state of ceremonial cleanness after some uncleanness infraction. This applied not only in the case of skin diseases such as leprosy, as you noted, but also in a wider variety of matters, such as eating certain kinds of unclean food and even taking the scapegoat out into the wilderness on the Day of Atonement. Since being ceremonially clean meant being able to participate in good standing in the worship of God, it was a natural development for John to apply the practice of washing to a more profound sense of being “clean,” that of being genuinely repentant for one’s sins.
As for why Jesus wanted to be baptized even though he had no sins to repent of, we need to realize that John’s baptism was not an end in itself. It was a means to an end. John’s message was, “Repent, for the kingdom of heaven has come near.” John didn’t want people to repent and be baptized just because they were sinful. He wanted them to do that because God was breaking into the world in a new way, and he wanted people to be able to join in what God was doing.
So, in effect, John was saying, “Anyone who wants to be part of what God is doing, come and be baptized.” The implicit assumption in the case of most people was that this would involve forsaking sins and having a new orientation in life. But for someone like Jesus, who already had the right orientation—he always wanted to be doing what his Heavenly Father was doing—and who had no sins to repent of, it was still appropriate for him to make the public gesture that John was calling for.
I think that is why Jesus told John to go ahead and baptize him “to fulfill all righteousness.” John was saying, “If you want to be part of the new thing that God is doing, come and be baptized.” By being baptized, Jesus was saying, “Yes, I want to be part of the new thing that God is doing!” He didn’t need to have sins to repent of in order to make that public declaration.