The snake was the new religion's answer to the old. When the earliest writers of the First Testament sought to differentiate themselves from the 'Pagan' religions that were dominant at the time, they chose a 'bad guy' that was reminiscent of their good-guys - the snake.
The snake character shows up in several ancient mythologies as a wise and sometimes powerful creature. By characterizing the same beast as a trickster or untrustworthy element, the early Jews created a dynamic in which the old religions could be seen as foolish and deceptive.
The snake in Genesis plays the role of the Devil, the fallen angel who had previously communed with God. His seduction of Eve is a metaphor for a sexual seduction as well as the mental 'brainwashing' of other competing religions. Finally, the snake can be understood in a post Aquinian sense as a 'necessary' catalyst for Man's development of 'will' as an essential aspect of his relationship with God.
