Read Genesis 4 at Bible Gateway.
And in the process of time it came to pass that Cain brought an offering of the fruit of the ground to the Lord. Abel also brought of the firstborn of his flock and of their fat. And the Lord respected Abel and his offering, But He did not respect Cain and his offering. And Cain was very angry, and his countenance fell. So the Lord said to Cain, “Why are you angry? And why has your countenance fallen? If you do well, will you not be accepted? And if you do not do well, sin lies at the door. And its desire is for you, but you should rule over it.” Gen 4:3-7
Apparently there was a right and wrong way to bring offerings to the Lord, and Cain and Abel both knew it. I say that, because first of all, later in Torah the Lord gives detailed instructions on the acceptable offering which makes the worshiper acceptable in His sight (Lev 1 and following). So we learn that there are acceptable and unacceptable offerings.
Second of all, the Lord says to Cain, after his offering was rejected, “If you do well, will you not be accepted?” In other words, “Did you desire for Me to accept your offering, Cain? Then bring an acceptable offering.” The boys had to know what was acceptable and unacceptable, or else the Lord’s response to Cain would have been unjust.
Now I began puzzling over this as I was meditating on this chapter, and the next one. Gen 4 tells us the genealogy of Cain’s seed. He and his seed were well known, Josephus tells us, for introducing, practicing, and perfecting evils in the earth. They were the unrighteous line. Gen 5 tells us the genealogy of Seth’s seed. Seth replaced Abel as the son of righteousness, and his seed were the righteous line.
Now how do two boys from the same parents, raised in the same house, go such different directions? If seed produces fruit after its kind, then how is it that there was a good tree which produced good fruit (Abel and Seth), and a bad tree which produced bad fruit (Cain), when both trees were from the same place?
Both boys were taught the same information and expected to perform the same duty and service. But here is the difference. One son, Abel, when he got up that morning, made a choice to do well, and one son, Cain, when he got up that morning, made a choice to not do well. Abel brought an acceptable offering in accordance with his choice, and Cain did not, in accordance with his choice.
Nothing bad happened to Cain right away, as a result. The Lord gave him a warning and some good advice. When we first choose to not do well, sin lies at the door. It is we, by our choice, who open the door, which then allows sin a place to crouch. Sin, crouching at the door, engages us in a battle of wills. It desires mastery, but we must not let it rule us … we must rule it. That is another choice.
The road to a failure as big as Cain’s, the murder of his brother, is lined with a series of small choices, each one building on the other. It begins with opening our eyes in the morning, and choosing, for this day at least, to either do well or not. At any time along the way, we can get off the road of unwise choices, by choosing at the next juncture, to do well. To master sin. To not let it be the master of us.