Gen 4:7 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.
According to Scofield, the word sin used in "sin lieth at the door" is the same word in the Hebrew for "sin offering." In conjunction with Romans 5:20, "where sin abounded, grace did much more abound" God was telling Cain that if he does not do well, or if he sins, grace in the sight of God is ever present! The grace of God is always there for the accepting, it is up to the offender to accept what the Lord is so willing to offer.
If you are lost, if you have never trusted Christ as your Savior and accepted the grace He offers for your sin; will you do so today?! This is the most important decision you will ever make. Grace is there for the taking! Will you accept what is freely offered?
If you are a Christian, if you have been born again; might there be something within your life that breaks fellowship with God and enjoyment of the Christian life? You too can be restored to a personal walk with Christ, and the joy of the believer simply by accepting the grace of God that bought you to begin with! Ask the Lord to help you, to forgive your straying, and come back to the place of fellowship with God!