We all seem to start out much like Cain. In Genesis, we see a man who knew that God required sacrifices to Him because of the Fall. However, Cain much like most humans wanted to do things his way and not God's way.
God being every merciful and full grace offered Cain another chance.
In the course of time Cain brought to the Lord an offering of the fruit of the ground, Genesis 4:3 (ESV)
but for Cain and his offering he had no regard. So Cain was very angry and his face fell. Genesis 4:5 (ESV)
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)
But I'm afraid that the story does not end well. Cain was not interested in another chance. In fact, he goes on to murder his brother. God gives him another chance, but the sad tale of Cain ends with a continual rejection of God's grace and will.
How like Cain we all tend to be, we give God only what goes along with what we want to do. However, as soon as God's will diverges from ours, we get angry, our faces fall, and we sink deeper into sin. We like Cain refuse God's unending grace; we refuse to be reconciled. We are no longer interested in doing well and sin is crouching at the door waiting for us to turn from God.
There is another side to the story in Abel. Abel acts on faith in God and trusts that God's way will reconcile him to the Father.
and Abel also brought of the firstborn of his flock and their fat portions. And the Lord had regard for Abel and his offering. Genesis 4:4 (ESV)
Just as God has slain the animals to have skins to cover Adam and Eve after the Fall, Abel gave a blood sacrifice. Note that he was not only obedient, he was also generous in his offering to God. Abel gave the firstborn or the best of his flock and the best parts of the sheep to God. Abel held nothing back, giving God his all. He chose to embrace the grace of the Father through faith and obedience. We are reminded in the new testament of Abel's faith.
Keep in mind that though each brothers choice of offering reflected the inner heart, it was the offering itself that was unacceptable or acceptable. Though it may seem to be harsh to us, Cain was not being obedient to God and thus his offering was refused. God's way is the only way to God. For us, Jesus is the only way to God.
By faith Abel offered to God a more acceptable sacrifice than Cain, through which he was commended as righteous, God commending him by accepting his gifts. And through his faith, though he died, he still speaks. Hebrews 11:4 (ESV)
Jesus said to him, "I am the way, and the truth, and the life. No man comes to the Father except through me." John 14:6 (ESV)
Furthermore, we see that the brothers, though a true historical event, serve as representation or metaphor for the choice we are given to be the child of God or the child of the devil.
Everyone who makes a practice of sinning also practices lawlessness; sin is lawlessness. You know that he appeared to take away sins, and in him there is no sin. No one who abides in him keeps on sinning; no one who keeps on sinning has either seen him or known him. Little children, let no one deceive you. Whoever practices righteousness is righteous, as he is righteous. Whoever makes a practice of sinning is of the devil, for the devil has been sinning from the beginning. The reason the Son of God appeared was to destroy the works of the devil. No one born of God makes a practice of sinning for God's seed abides in him, and he cannot keep on sinning because he has been born of God. By this it is evident who are the children of God, and who are the children of the devil; whoever does not practice righteousness is not of God nor is the one who does not love his brother. For this is the message you have heard from the beginning, that we should love one another. We should not be like Cain who was of the evil one and murdered his own brother. And why did he murder him? Because his own deeds were evil and his brother's righteous. I John 3: 4-12 (ESV)Right on, John! I certainly can't say it much better than he did. Cain chose to be a child of the devil. He choose sin over God's redeeming grace. Though we may continue to struggle with sin, we will not continue to practice sin if we are to be Abel's. God's seed in us make it impossible for us to continue to live in sin. I know we all fall down from time to time because we are after all human, but we will choose to turn to God for forgiveness and life change when we are born of Him.
Who do you choose to be? Cain or Abel? A child of the devil or a child of God?