So, Cain, the little man brought forth into the world with God’s help, and little Abel, the frail boy whose life held no meaning for his parents, and to his brother, grew up side by side. They learned to till the hard and unyielding ground, and to tend the trusting creatures around them; and to kill them at regular intervals, so they might wear their hides. We are to understand the years passed, time continued, life unfolded.
Then, the third sentence in Genesis chapter 4 shifts the scene to a much later time.
“Now Abel was a keeper of sheep, and Cain a tiller of the ground.”[1] Did Adam and Eve tell the boys how wonderful it had been in Eden? But now Adam and Eve had to work by the sweat of their brows, farming and taking care of domesticated stock, since that was God’s command to them back in Genesis 1. When the boys grew up they had to support themselves, Cain went into the agricultural end of the family business, and Abel went into ranching and herding.
It seems there was a third, major, aspect of life Adam and Eve inculcated into their sons: bringing something of their livelihood back to God. Long before there were laws, and rituals, and religious rites, there was an altar and there was a sacrifice. We read that “in the course of time Cain brought to the Lord an offering of the fruit of the ground, and Abel for his part brought of the firstlings of his flock, their fat portions.”[2]
There are two things to see here: First, this phrase “in the course of time,” in the Hebrew, actually is “at the end of days,” suggesting there was a prescribed time to make offerings. Maybe it was once a year, “at the end of days,” indicating, for example, at the end of the growing season, when harvest was about to begin. The phrase “brought to” makes it seem as though Cain and Abel went to a place with their offering – they were to bring it before the Lord, maybe a place where they were to appear in the presence of the Lord. Some have suggested that place might have been where God had set the cherubim and swirling, flaming sword, the very gateway to Eden itself, the beautiful land now forbidden to them, the cradle of their beginnings, the fading memory of all that was beautiful, and pure, and good.
As an interesting aside, depictions of enormous winged creatures were often used to guard the gates of ancient cities and buildings in the middle east, and when God gave the pattern to Moses for the ark of the covenant, on the lid were two cherubim with their wings outstretched, over what God called the mercy seat. Once a year, at the apex of the Day of Atonement the high priest would bring into the Holy of Holies, to the mercy seat of God, the blood of a lamb sacrificed for all the people. Maybe groundwork had been laid down in the days of Cain and Abel for explaining later about blood sacrifice, and the sprinkling of the mercy seat, with this earliest scene of Adam and Eve and their children coming once a year to the gateway of Eden, with an offering for the Lord.
The offerings Cain and Abel brought to God reveal a significant difference between the two men. Cain’s offering of fruit from his fields was instantly rejected; but Abel’s offering of a lamb was instantly accepted. The Bible doesn’t say how God showed His acceptance of Abel’s offering, but in the book of Judges, when Gideon made an offering up to God, a supernatural fire came from heaven and consumed it instantly; the same thing happened later for the prophet Elijah in a hair-raising story regaled in Israel’s history. Whichever way God showed His favor to Abel and His disfavor to Cain, it was obvious. Cain knew it, and Abel knew it.
The ancient Israelites, who first wrote down these stories, would have immediately recognized the problem with the Cain’s offering. Even though there were grain offerings and wine offerings, they understood that a person could not approach God without the shedding of sacrificial blood. Whether Cain understood this is conjecture. What was really more wrong with Cain was not what he offered, but the condition of his heart when he offered it. God was not pleased with Cain because unrepentant sin was already in his life.
Cain’s character was revealed in his offering. He gave only “some” of what he had, there iss no sense of his offering being from the best of what he had, or that it cost him anything. Instead it seemed like a casual approach to a chore, as though Cain brought what was convenient, so indifferent towards God, that he brought the Lord the leftovers. Cain thought giving God some of his own good deeds would suffice – he didn’t give the best and first of himself.
Untold millennia later, the New Testament author of the book of Hebrews wrote, “By faith Abel offered to God a more acceptable sacrifice than Cain’s. Through this he received approval as righteous, God himself giving approval to his gifts; he died, but through his faith he still speaks.”
 
[Andrey Mironov [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
[1] Genesis 4:2b (NRSV)
[2] Genesis 4:3-4 (NRSV)