First of all, the Hebrew word for repentance is teshuvah. As I have pointed out (http://practicaljesus.blogspot.com/2009/02/teshuvah.html), the "baptism of repentance" is better understood as an "immersion in teshuvah."
The concept of immersion (greek baptizo) could be used to mean the traditional immersion in water, but it was also an idiom which meant a complete and utter dedication to a course of action, or duty. In Matthew 20:22 Yeshua used the term in this way.
"... Yeshua said, You do not know what you ask. Are you able to drink the cup which I am about to drink, and to be baptized with the baptism with which I am to be baptized?..."Likewise at Luke 12:50, Yeshua used the term in this way again.
"But I have a baptism to be baptized with; and how am I besieged till it be accomplished!"In other words, a complete and utter surrender to the performance of duty (in this case equity) in all interpersonal relations. For what reason? For the "remission (or release) of sin."
It is important, however, to understand that this act of baptism, contrary to modern Christian belief, was not intended to release the one baptized from sin, but the one for whom the act of equity was performed. In other words, Yeshua believed that a community focused on performing teshuvah (acts of equity) would experience a release from the ravages of sin and a return to a state of shalom.