To answer your question, baptism was a common Jewish practice long before it became a Christian practice. The basic Hebrew meaning of baptism is identification. When one was baptized he identified himself with a person, and/or message, and/or group. In Judaism, when a Gentile underwent the process of conversion to Judaism, he would also be baptized and, by so doing, would identify himself with Judaism and the Jewish people. Those who were being baptized by John were identifying themselves with John's repentance movement and with the message he was proclaiming concerning the soon coming of the King. By being baptized by John, they were making a commitment that whomever John declared Messiah to be, they would accept Him as the Messiah.