Cain said to Abel his brother, “Let us go out to the field.” And when they were in the field, Cain rose up against his brother Abel, and killed him. Then the Lord said to Cain, “Where is Abel your brother?” He said, “I do not know; am I my brother’s keeper?” And the Lord said, “What have you done? The voice of your brother’s blood is crying to me from the ground. And now you are cursed from the ground, which has opened its mouth to receive your brother’s blood from your hand. When you till the ground, it shall no longer yield to you its strength; you shall be a fugitive and a wanderer on the earth.”
Here is where the Lord first indicated that His approval would require a blood sacrifice for man to become positionally acceptable in His sight. The issue of blood was significant of a future event when He would sacrifice His own Son on the cross as expiation for the sins of all mankind. Had Cain traded with his brother for an animal to sacrifice he would have received God’s approval also, but he was determined to meet God on his own terms with contemptuous arrogance and by so doing displeased the Lord. As a result Cain became intensely envious of Able which led to malice and culminated in man’s first act of murder.