Genesis 4 is an interesting chapter because it is the first glimpse of a sinful world after the Fall. Genesis 4.7 gives us a great deal of information about sin. We read as God speaks to Cain after his sacrifice was not accepted: If you do what is right, will you not be accepted? But if you do not do what is right, sin is crouching at your door; it desires to have you, but you must master it.
What is striking about this picture is that sin is active - it desires to have you, it is crouching at your door. That means you always have an enemy hiding, prowling, leaping - ready to attack. We do not even have to go and look, sin is right there on the doorstep.
What does this mean? It means that there is no down-time for a believer in Jesus. If sin is right there we must be ready to stand firm. It also means that the believer must take sin seriously - it is not like a rabbit attacking but as we read in the New Testament, a lion seeking to devour God's people.
Sadly, Cain was mastered by sin as he invited his brother out to the field and he killed him. He was mastered by sin and it led to devastating consequences.
Do not let sin master you instead master sin.