Genesis 4:6 So the Lord said to Cain, “Why are you angry? And why has your countenance fallen? 7 If you do well, will you not be accepted? And if you do not do well, sin lies at the door. And its desire is for you, but you should rule over it.”

The first manifestation of the fall of man was seen when man hide among trees covering their nakedness with leaves. The second one was seen when Cain killed his brother Abel. However, a careful look at the conversation of Cain and God show us something very important about sin.

After the fall of man, God had initiated a process of atonement on the earth. He showed the pattern to Adam and Eve after He clothed them, killed an animal, shed the blood for their atonement. God covered them with the animal skin as a proof of their recipient of imputed righteousness.

Hebrews 9:22 And according to the law almost all things are purified with blood, and without shedding of blood there is no remission.

Cain knew that it is through atonement that sins are forgiven, but he decided to bring fruit of his own labour instead of atonement by the blood. God simply said, "if you do what is right, will you not be accepted? So also today, you cannot be saved from the power of sin and guilt without the sacrifice of Christ Jesus.