Question: Doesn't the story of Cain and Abel defeat Calvinism? "The LORD said to Cain, “Why are you angry, and why has your face fallen? If you do well, will you not be accepted? And if you do not do well, sin is crouching at the door. Its desire is for you, but you must rule over it.”" -Genesis 4:6-7 (ESV) See God tells him if he does something he will be accepted.
Response: "If you do well" is a phrase with a verb in the subjunctive mood - a conditional statement which asserts nothing indicatively. "if you are willing", "if you hear", "if you do" declare, not man's ability, but his duty - what he OUGHT to do but such statements say nothing of what he CAN do.
While it is true that if Cain did well he would be accepted and that he had a duty to resist sin. But that doesn't mean that he had the moral ability to do so.
The passage also contains the command, "...you must rule over it." What does the Bible teach about the purpose of Divine commands? According to Paul in Romans 3:19-20, the purpose of imperatives (commands) are to reveal sin, man's inability, and NOT his ability to do what he is commanded. It reveals his impotence and desperate need of grace. This is the reason the Bible ITSELF gives for God commanding us to do things we we are incapable of doing. Therefore any other conclusion is man extrapolating using only his own human reason to conclude that we must have the ability if God commands it. In this case human reason directly contradicts God's word.
So God commanded Cain to do something he was unable to do and the whole time God knew he couldn't do it?
Every command in the Bible is given by God to a people He knows cannot and will not obey him. (Read Rom 3:19-20). This principle can be seen in every day life in many ways . For example if someone borrows a huge sum of money from the bank and then squanders it in Vegas, his inability to repay the bank does not alleviate his responsibility to do so. The bank still requires it of him. Likewise, in Adam, we owe a sin-debt that we cannot repay. Our inability to repay it, or even keep His commands does not change God's holy requirements. God is holy ...we fell ... God is still holy so His holy commands do not change to accommodate our sinfulness. It would violate His essential character to do so. Instead, in love, He came as Jesus to do for us what we were unable to do for ourselves.
If you still doubt this concept, ask yourself, are you morally able to obey all of God's commands, or do you need Christ? What does the Bible say about that? Since all people are sinners and cannot obey God's commands, and God knows it (Ps. 14:3, Is. 53:6; John 3:19-20; Rom. 3:11) and since Cain shares the same fallen nature with you and I, then it follows that, yes, God gives commands to Cain which God knows he cannot obey, as He does all who in Adam. The fact that God commands something of us that we are unable to do is the very reason Jesus Christ came to this earth to rescue us. This question, therefore, is intimately associated with the very gospel itself. Thank God for His great mercy in Jesus Christ for all the believing ones. Their sins are forgiven for Jesus' sake.