From the Book of John Chapter one. we know that Jesus was right there with Cain and Abel. That's why it's so important to understand the role of Jesus in the lives of Cain and Abel!
Genesis 4:3 "And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the Lord. 4 And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the Lord had respect unto Abel and to his offering: 5 But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell. 6 And the Lord said unto Cain, Why art thou wroth? and why is thy countenance fallen? 7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him. 8 And Cain talked with Abel his brother: and it came to pass, when they were in the field, that Cain rose up against Abel his brother, and slew him. 9 And the Lord said unto Cain, Where is Abel thy brother? And he said, I know not: Am I my brother's keeper? 10 And he said, What hast thou done? the voice of thy brother's blood crieth unto me from the ground."
Cain was the oldest son and rightful heir to Adam. Hence, Cain had the "birthright", called progeniture in ancient law. The eldest child was to inherit the entire estate of both parents. Upon the demise of the parents by progeniture, the eldest son became head of the estate and basically a surrogate parent to the younger children. Cain was by birthright to be esteemed over Abel.
God, we find from the New Testament is no respecter of persons (Acts 10:34). Although "birthright" is important to the eldest, it has little prestige in the eyes of God. It is to be remembered that the birthright carries with it three characteristics: 1) Pride in the position and 2) Wealth in having that position, and 3) the parent's blessing favoring the eldest.
If we zoom forward a millenneum we find that Esaw gave up his birthright:
Genesis 27:36a "And he (Esaw) said, Is not he rightly named Jacob? for he hath supplanted me these two times: he took away my birthright; and, behold, now he hath taken away my blessing.
Isaac blessed Jacob, just as God blessed Abel. What's more as we can see from Hebrews, Cain,s heart was wrong just as was Esaw's:
Hebrews 12:16 "Lest there be any fornicator, or profane person, as Esau, who for one morsel of meat sold his birthright."
Cain had a "pride" problem, just like Esaw! God honored Abel's offering, but not Cain's! Cain was humiliated because in his position as eldest, he "should have been" more respected, but he wasn't! He was prideful just as were his parents back in the Garden of Eden. Just as Adam and Eve were punished by God, so was Cain. Pride always separates man from God.
Now let's look at the offerings. Abel offered "the firstlings of his flock and of the fat thereof" and God respected that. Zoom ahead again.
"Offerings" mean "bring forth or bring near" and in early Hebrew meant "sacrifices". Hence, Cain and Abel were making a sacrifice to God. Sacrifices were of three main types: 1) blood offering, 2) grain offering and 3) drink offering. Hebrews tells us also about Abel's offering (sacrifice):
Hebrews 11:4 4 By faith Abel offered unto God a more excellent sacrifice than Cain, by which he obtained witness that he was righteous, God testifying of his gifts: and by it he being dead yet speaketh." (Yes, Abel's spirit spoke from the grave!)
Abel's sacrifice was offered "by faith" implying that Cain's sacrifice was not accompanied by faith, but was ritualistic in nature. He did it because that was "the law"; it was God's expectation.
One thing to be understood about faith is that it points toward Jesus! A demonstration is "the faith of Abraham".
Faith is what's right! It is credited to the faithful as righteousness.
Romans 4:13 "For the promise, that he should be the heir of the world, was not to Abraham, or to his seed, through the law, but through the righteousness of faith."
Therefore Abel's offering was more excellent because it was a righteous offering given in faith.
Romans 4:16 "Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all..."
We can see from Romans 4 preceding that "faith" is accepting God's "grace". Forgiveness is God's free gift. It wasn't the sacrifice that made the difference, but the condition of the heart. Abel's sacrifice was by faith and Cain's was that "which is of the law". Faith and grace always point toward Jesus. Somehow, someway God let our antecedents know that there's more to come than "this"! It is apparent that since Jesus walked in the garden in the cool of the evening with Adam and Eve, that Cain and Abel knew of Jesus!
The sacrifice itself was important, but not because of its material, but by the understanding of the one making the sacrifice. First off, neither offering was perfect. Levitical law, to come later, required a meat offering accompanied by a grain offering (except on special occasions). The meat offering was a sacrifice in atonement for sin and the grain offering was a showing of love.
Leviticus 14:19 "And the priest shall offer the sin offering, and make an atonement for him that is to be cleansed from his uncleanness; and afterward he shall kill the burnt offering."
Leviticus 1:1-17 tells of a proper burnt offering. It's the dressed meats containing the fats burnt on an altar by the priest for atonement from sin while the blood of the animal is sprinkled around the altar.
What cleanses man from sin?
Hebrews 9:22 "And almost all things are by the law purged with blood; and without shedding of blood is no remission."
We are told several places in the New Testament that Abel was righteous. His faith was in the blood and the blood pointing toward Jesus. It took the "blood" sacrifice of the firstfruits of the flock as a remission for Abel's sins. Cain offered no blood! He was a tiller of the ground you say! Bartering grain for an animal addresses that issue! It takes "blood" to be an acceptable sacrifice. Abraham had a problem too. He had no animal sacrifice, but said "God will provide!" Abraham had faith and God did provide the kid for the sacrifice. Cain hadn't the faith of Abraham!
The flesh and blood is a picture of Jesus hanging on the cross. The animal sacrifice was a picture of Jesus sacrifice on the cross. Just as our antecedents sprinkled blood around the altar, Jesus's blood sprinkled around the cross. He was the royal priest who sacrificed himself for our sins. That's grace! Faith is having confidence in the shedding of blood, innocent blood. Jesus's blood. That's God's grace and that was the faith of Abel!
Cain suffered the consequences of his lack of faith:
Genesis 4:10b says "the voice of thy brother's blood crieth unto me from the ground". Cain's guilt was announced from the grave. God knew what Cain did and the condition of his heart! The ground cried out for justice! We see from Luke that "the very stones would cry out" for justice. The very ground would cry out for Jesus. Luke 19:39 "And some of the Pharisees in the crowd said to him, “Teacher, rebuke your disciples.” 40 He answered, “I tell you, if these were silent, the very stones would cry out.” Perhaps Jesus paraphrased what he had said to Cain back in the beginning. Cain was a type of Pharisee and Jesus said nearly the same thing to the Pharisees!
Abel was a "type of Christ". Abel was a righteous man innocent of a crime who died unjustly. His blood was shed for righteousness sake just as was the blood of Jesus. While Abel wasn't perfect as Jesus was, his sacrifice was by faith in the blood. Not his own, but the firstflock. Jesus was the perfect and only son of God, the first fruit:
1 Corinthians 15:20 "But in fact Christ has been raised from the dead, the firstfruits of those who have fallen asleep. "
Yes, the firstlings of the flock points toward the resurrection of Jesus! Paul taught that faith in Jesus included faith that he yet lives! Faith in the resurrected Jesus, the firstfruit of the dead.
How did Cain sin? There was yet no written law, but Cain knew! His father talked with Jesus. Jesus gave the "Greatest Commandment": love god and love others! Cain broke both of those! He failed to love God by making a prideful sacrifice and he failed to love Abel because he was envious and out of pride, committed murder. Cain knew God's heart, but he failed miserably! He broke the commandment not to be written until 3000 years or so later! Who says Jesus wasn't there?
It astounds me when I read about mankind's start in this world. God knew we would fail and need Jesus. He knew that all along and planned for it! Cain and Abel knew God's expectations. Abel obeyed. Cain rebelled. We all know God's expectations. Some have faith; others don't! Abel took the narrow path while Cain took the wide. Abel took the path less taken. Cain took the selfish route, the one most traveled. He was just as the rich man in the New Testament who just couldn't follow Jesus because of his wealth. Cain just couldn't do that either. He gave scraps because his firstfruits just meant to much to him. What about you? Are other things more important than God?