"Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother's righteous." (1 John 3:12)
These two brothers stand as contrasting prototypes. Cain was the first child born after the Fall who embraced the "wicked one" in spite of all the firsthand and face-to-face knowledge of God's redemptive plan and offering of grace (Genesis 4). Cain's arrogant lifestyle is noted in Jude 8-11. Abel, in contrast, was a man of great faith (Hebrews 11:4) who was both righteous (Matthew 23:25) and a prophet (Luke 11:50-51).
Adam and Eve would have taught the boys (and their other children) about God and the knowledge of the sacrifice (covering of skins) for their own sin. It is clear that sheep were not kept for food (Genesis 2:16) since Cain provided food (as instructed by God--Genesis 1:29). Abel provided clothing and sacrifice.
The events of the Fall would suggest that this sacrifice was an established practice (Genesis 3, the "covering" of skins--the Hebrew word for atonement means "to cover"). Furthermore, the language of Genesis 4:3 (Hebrew translation: "at the end of the days") requires a specified time period when they brought (Hebrew translation: "came with"), probably to the door of the garden (Genesis 3:22-24), an offering (used consistently of voluntary tributes to God, Exodus 30:9-10). It is completely parallel to "the firstlings of his flock and of the fat thereof" as later used by Moses in Leviticus 9:3-10.
Such specified action is hardly accidental. Thus Cain's rebellion and heinous fratricide revealed an evil heart that would not repent. May God protect us from such evil. HMM III