Mark 1:4-5

4John the Baptist appeared in the wilderness preaching a baptism of repentance for the forgiveness of sins [that is, requiring a change of one’s old way of thinking, turning away from sin and seeking God and His righteousness].

5And all the country of Judea and all the people of Jerusalem were continually going out to him; and they were being baptized by him in the Jordan River, as they confessed their sins.