Cain, so far as the Biblical record is concerned, was the very first human being ever born. He was the first to come out of the womb; the first person to be created by sexual union. Adam was formed from the dust, and Eve from Adam’s rib. Both were created mature (so far as we know) so they did not know the travails of infancy and childhood as every generation after them would. They did not know experientially what awaited them.
It is entirely possible that Eve believed Cain was the promised seed from Genesis 3:15, the deliverer who would crush the serpent’s head and set things right. Cain’s name means “acquisition.” Whatever the case, Eve was grateful for the gift of a child and named him thusly.
2 And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
Abel’s name means “vanity” or “vapor.” Clearly Eve was discontent with the acquisition of a second son, or perhaps discontent that the effects of sin upon God’s perfect creation were beginning to be more easily recognized. Abel became a keeper of sheep, perhaps for the benefit of the wool and perhaps so that he could have a gift of offering to God when the Lord commanded it.
Cain’s occupation was to till the ground that God had cursed, Genesis 3:17-19. In that regard Cain worked with the very dust that his father had been taken from, 1st Corinthians 15:47.
3 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.
Here we have the implication of localized worship. God had apparently designated a place where the worshiper could draw near to Him and offer the sacrifice He commanded. A period of time had elapsed, at least enough time for both brothers to enter into manhood and their careers. God set the standard in Eden with the sacrifice of an animal to clothe Adam and Eve, Genesis 3:21. Blood sacrifice, presumably symbolized by the lamb, was what God commanded. Cain instead brought the fruit of the cursed ground to God as an offering, perhaps thinking that the labor of his work would satisfy Him.
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:
Abel brought to the Lord the best of his flock and their fat as an offering. The Lord respected Abel’s offering. What does this mean? It simply means He accepted it. The offering of the finest of Abel’s flock was the substitutionary sacrifice that permitted a worshiper to approach a holy God, for the blood of the substitute covered their sins. Abel trusted in God’s provision (blood atonement) while Cain seemingly trusted in his own merit (the fruit of his husbandry).
5 But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.
Cain’s reaction was entirely natural for our sinful nature; he became angry. In fact the wording used suggested a seething anger bordering on rage. His countenance fell, as in his outward appearance reflected his inner turmoil. In other words Cain was not one of those who could expertly hide his emotions. His heart was on his sleeve, as the saying goes. It becomes clear that Cain was not angry at himself at this point. He was likely upset with his brother for doing what he had been told, and he was upset with God for rejecting his offering. As Jesus said, murder begins in the heart of man and the act of murder is only a manifestation of what the heart already longed to do, Matthew 15:19-20; James 1:13-15.