John the Baptist in scripture is called the "forerunner of Christ."  His ministry marked the turning point in redemptive history.  Prior to that, the great truths of Christ and His kindgom were veiled in the types and shadows of the law, and promised in the writings of the prophets.  But John the Baptist introduced the King Himself by proclaiming, "Behold the Lamb of God, who takes away the sin of the world!" John 1:29. 

John's baptism was a washing, symbolizing moral regeneration, administered to each candidate only once. He criticized the people for presuming to be righteous and secure with God because they were children of Abraham (Matt 3:9). John laid an ax to the root of this presumption. He warned that they, the Jews, would be purged and rejected unless they demonstrated fruits of repentance (Matt 3:7-12).

John's effort at moral reform, symbolized by baptism, was his way of preparing Israel to meet God. He began his preaching with the words, "Prepare the way of the Lord, make His paths straight" (Mark 1:3). He had a burning awareness of one who was to come after him who would baptize in fire and Spirit (Mark 1:7-8). John was a forerunner of this mightier one, a herald of the messianic hope which would dawn in Jesus.
(from Nelson's Illustrated Bible Dictionary, Copyright (c)1986, Thomas Nelson Publishers)