If you do well, will you not be accepted? And if you do not do well, sin is lurking at the door; its desire is for you, but you must master it.
(The Holy Bible: New Revised Standard Version (Nashville: Thomas Nelson Publishers, 1989), Ge 4:7.)
God accepted Abel’s sacrifice, but not Cain’s. Who knows why? Any guess is just a guess.
God is dealing with Cain about a heart issue. Cain has a choice to make. He is a free human being, able to decide. God advises him to master the sin that is trying to overtake him.
Cain is not to give in to this lurking sin. He is to master [timšol] it. The sense of the Hebrew form (2nd masc. sing. imperfect) is ambiguous; it may be read as a promise (“you shall master it”), as a command (“you must master it”), or as an invitation (“you may master it”). Although each of these is quite possible, notice that Cain does have a choice. He is not so deeply embedded in sin, either inherited or actual, that his further sin is determined and inevitable. The emphasis here is not on Cain as a constitutional sinner, one utterly depraved, but on Cain as one who has a free choice. When facing the alternatives, he is capable of making the right choice. Otherwise, God’s words to him about “doing well” would be meaningless and comic. Should he so desire, Cain is able to overcome this creature who now confronts him. The text makes Cain’s personal responsibility even more focused by its use of the initial emphatic pronoun: “you, you are to master it.
(Victor P. Hamilton, The Book of Genesis, Chapters 1–17, The New International Commentary on the Old Testament (Grand Rapids, MI: Wm. B. Eerdmans Publishing Co., 1990), 228.)
I want to do a better job of resisting sin than Cain did. I have a choice too.