(Hebrews 11:4 By faith Abel offered unto God a more excellent sacrifice than Cain, by which he obtained witness that he was righteous, God testifying of his gifts: and by it he being dead yet speaketh.).
It also tells us that Cain was jealous of Abel. God told Cain that because of his jealousy sin lay at the door of his heart. In time, Cain became angry and killed Abel, and Abel's blood cried out to God from the ground. Eve surely must have then felt sorrow over her children. God confronted Cain with his sin, and Cain denied his deed by asking "Am I my brother's keeper?" But God knew what he had done, and he punished him by sending him into the world away from his family, to be a wanderer and never have a home of his own. The ground would never yield for him again. He was to be hidden from the presence of God, and he cried out that his punishment was too great to bear. God put a mark on Cain to let the rest of the world know what Cain had done, and to keep them from killing Cain as judgment.