"If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him." Genesis 4:7
"...sin lieth at the door..." When I was growing up, my parents were constantly telling us to "close the door we were letting the bugs in (mosquitoes, flies, etc.)." What a spiritual principal we find in this natural illustration. According to our opening scripture, sin lies at the door just like those bugs. If we didn't want the bugs to come in we had to keep the door closed or close it before they had a chance to come in. Same thing- if we don't want sin to come in we should either keep the door closed or close it quickly.
"...sin lieth at the door..." It "lies" at the door. It isn't standing at the door, it isn't pacing back and forth around the door, it isn't sitting down at the door, it isn't watching the clock wondering when you will open the door. It's "lying" at the door. That means that it has taken a comfortable position and is just waiting. It isn't going anywhere, it's content to just lie there and wait until you open the door. It "lies" at the door- it's also laying in wait. Waiting to "pounce" on you. Waiting as a lion lays in wait for it's prey. It's lying at the threshold so that at the first sign of movement, the first sign of the door opening it will make it's attack on you.
Since it's obvious that sin is going to wait you out and at the first sign of an open door it will make it's move to destroy you, then the only victorious course of action for us to take would be to keep the door closed. It's a whole lot easier to keep sin out than it is to deal with it once it has come in- just like those bugs. (It was an endless battle swatting flies and killing mosquitoes.) How do we do that? How do we keep the door closed? "If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door." Do what's right--- keep the commandments, obey the Lord, apply His word to your life. As long as you are living in obedience to God's word you will keep the door closed and sin won't have a chance to enter.
Have a great day. Close the door, you're letting "sin" in.