Genesis 4:7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto shall be his desire, and thou shalt rule over him.
The Lord confronted Cain and gave him a chance to repent and be accepted. He asked Cain, ”Why are you angry and your face fallen? If you do the right thing you will be accepted; if you do the wrong thing greater sin and misery will come your way.” The right thing (“doing well”) is repentance and trust in mercy; the wrong thing (“doing not well)” is resisting repentance and faith.
Cain responded by choosing not to ”do well.” He did not repent and turn to the Lord, and, well, we know the rest of the story. Repentance and faith in the Promise (Jesus) would have prevented the first recorded crime after the Fall. Every time we sin (which is every day) we have the opportunity to repent and come back into God’s good graces. For this reason, our life is one of continuous repentance and faith and renewed life. By this proper response we ”rule over him (Sin).”
Sin is described in this verse as a person: ”crouching at the door, his desire, rule over him.” This person is also described as someone we can ”rule over.” A real war is being waged for our soul, but faith in Christ puts us on the winning side. Death is swallowed up in victory and we rule.