The snake is an ambivalent symbol in Israel's world, associated with both life and death (see Numbers 21:4-9), as is the word "crafty" (compare to "sly fox"). Traditionally, the snake has been identified with the devil (first evident in Wisdom of Solomon 2:24), but it is doubtful that the writers of Genesis 3 had this idea in mind. The text itself does not make this identification, calling the snake a "wild animal that the Lord God had made" (3:1), the same word for "animal" that is used in 2:19. The snake is a part of God's good creation!
It seems best to understand that the snake is a metaphor, representing anything in God's creation that could present options to human beings, the choice of which could lead them away from a true relationship with God. The snake facilitates the options that the tree of knowledge of good and evil presents (2:16-17). While the snake does not tell the whole truth to the human beings (did he know it?), neither does God (see the "God knows" of 3:5). It cannot be claimed with certainty that the snake lies (see the NIV translation of 3:4, "you will not surely die" [emphasis added], and the additional move God must make in 3:22 for death to be certain).
Notably, God does not accept Eve's excuse that the snake had tricked her (3:13); human beings cannot escape responsibility by claiming that "the devil made me do it." Nevertheless, God passes judgment on the snake (3:14-15) for being involved in the presentation of options. Indeed, God is not "off the hook" for what happens; God made a command regarding the tree, making possible both the salutary and the devastating effects of human choices.
What is the nature of the sin committed by both Adam and Eve? It should be noted that Adam is just as guilty as is Eve; he was standing with Eve during the entire conversation with the snake (3:6) and made no effort to stop things! While they are disobedient to the command of 2:16-17, that is symptomatic of a deeper issue, namely, mistrust that God has their best interests at heart in the making of the command.
The story of Genesis 3 is often referred to as the story of the "fall" of humankind, a word that the story itself does not use. In view of the severe disruption of relationships that follows upon the sin, the word may best be understood as a "falling out" or a "falling apart." All the important relationships that the human beings have are disrupted, including relationships with God, with each other, with the animals, and with the ground.
3Now the serpent was more crafty than any other wild animal that the Lord God had made. He said to the woman, Did God say, You shall not eat from any tree in the garden? 2The woman said to the serpent, We may eat of the fruit of the trees in the garden; 3but God said, You shall not eat of the fruit of the tree that is in the middle of the garden, nor shall you touch it, or you shall die. 4But the serpent said to the woman, You will not die; 5for God knows that when you eat of it your eyes will be opened, and you will be like God,* knowing good and evil. 6So when the woman saw that the tree was good for food, and that it was a delight to the eyes, and that the tree was to be desired to make one wise, she took of its fruit and ate; and she also gave some to her husband, who was with her, and he ate.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org