Adam and Eve had two sons. The first son was named Cain and the second son was named Abel. Cain grew up to become a farmer and Abel grew up to become a Shepard. Adam and Eve were asked to sacrifice a lamb for disobeying God and they told this story to Cain and Abel. Abel was very concerned that his sacrifice be special to God, so he offered him his first and best lamb. Cain offered the Lord his straw. The Lord respectably took Abel's offering and not Cain's. Cain became very jealous and angry with his brother, so Cain asked Abel to go for a walk and he struck his brother down and killed him. Cain then heard the Lord's voice asking where his brother is. Cain responds "I know not: Am I my brother's keeper?" God told Cain that when he tries to cultivate the ground, it would no longer yield its best for him. Cain tells the Lord how horrible he felt and he was afraid that he would be killed. So God put a special mark up on Cain; if he were killed, his death would be avenged several times. Cain was then left to wander into the Land of Nod.
This is the first murder that occurred in the bible and it all began in Eden. Its immediate effect is to demonstrate the snowball effect of sin. Adam and Eve sinned against God and were cursed. The curse was passed on to their children. Early Christian tradition focused on Abel as the head of a line of prophets who were killed and emphasized his innocent blood; thus they set the framework for the theory that related Abel's innocent death to that of Jesus and saw Cain as representing the children of the devil.