In John 3:5, Jesus tells Nicodemus that to enter the kingdom one must be "born of water and the Spirit". What does to be born of water mean?
Mat 3:11 (NIV) (John the Baptist:)“I baptize you with water for repentance. But after me comes one who is more powerful than I, whose sandals I am not worthy to carry. He will baptize you with the Holy Spirit and fire.
Acts 3:19,20 (NIV) "Repent, then, and turn to God, so that your sins may be wiped out, that times of refreshing may come from the Lord, and that he may send the Messiah, who has been appointed for you—even Jesus."
To be born of water means to turn around from ones old carnal life and live a new life of repentance.
The main symbolic event in the Old Testament that covered this was the crossing of the Red Sea.
When John the Baptist decreased in importance, and eventually left the scene, the praxis of baptism was taken over by the Christian Church.