In the Bible, and in Christian tradition, the acceptance of God is often marked by the act of baptism. Amongst other things, it is meant to symbolise a fresh start.
I recently read a study which examined the link between washing and morality. The conclusion was that the two are very much intertwined in the human brain. Participants who were asked to think of previous unethical behaviour were significantly more likely to desire physical cleansing.
So i guess it's of little surprise that when we (the human race) devised an act to signify the removal of sin, and the starting of a new chapter of life, that water is involved.
