What caused Cain to be brutally angry and to look so sad and despondent? Was this merely a temper tantrum over his offering not being accepted? Was it jealousy because his younger brother found greater favor and acceptance in God's eyes? Why would an incident like this hold such tremendous gravity in Cain's mind? Why was the rejection of his offering so distressing to him—distressing enough that he was willing to commit murder—and then lie to the all-knowing God? Why did this event turn his world upside-down and cause him to lash out so violently?
Interestingly, where Genesis 4:3 reads, ". . . in the process of time it came to pass," the most literal translation is, "it came to pass at the end of days," meaning "at an appointed time." It is possible, then, that this may have been a Sabbath or holy day offering.
The account of Abel's faith in Hebrews 11:4 adds to the story:
By faith Abel offered to God a more excellent [acceptable] sacrifice than Cain, through which he obtained witness that he was righteous, God testifying of his gifts; and through it he being dead still speaks.
Romans 10:17 instructs that "faith comes by hearing, and hearing by the word of God." If Abel offered something by faith, it means he followed the words or instructions that came from God. The fact that Abel's sacrifice was "acceptable" while Cain's was not suggests a standard by which these sacrifices were judged. Thus, Hebrews 11:4 means that Abel was instructed on what sort of offering was appropriate, and by following those instructions by faith, his offering was accepted, and he was declared righteous. The fact that God rebuked Cain means that he, too, knew what was required but for some reason chose to ignore it.
A couple of possibilities exist regarding what instructions Cain and Abel had been given to define what was acceptable and required. First, God may have instructed Adam, Eve, Cain, and Abel about the same sacrificial system He added to the Old Covenant in Moses' day. The Bible records that both Noah and Abraham made burnt offerings (Genesis 8:20; 22:2, 13), a specific class of offering that represents a man's wholehearted devotion to God. In addition, Jacob made a drink offering on the pillar of stone that he raised (Genesis 35:14). From these examples, some sort of understood sacrificial system undoubtedly existed long before the specifics were recorded in Exodus—Deuteronomy.
A second possibility is that God did not instruct Adam and his family in exactly the same way as He did the Israelites but gave them enough to recognize the need for an animal sacrifice, whether in worshipping God or in symbolizing the future sacrifice of Christ to remit sin. From the examples prior to the Old Covenant, it is evident that they had some understanding of sacrifices, when they were to be made and what they symbolized. It is unlikely that men would have conceived the concept of offering animals or grain on their own, and even if they had, it is even more unlikely that God would have accepted any addition to the worship He specified. Such instruction must have come from God if He would accept it.
We may not know exactly why Abel made the offering he did or precisely what Cain knew to do but ignored. Yet, we can generally understand what was happening by remembering why God instituted sacrifices in Moses' time: They were added to the Old Covenant to remind the people of their sins—of falling short of God's glory (Galatians 3:19; Jeremiah 7:22-24). Whatever the exact infraction, something about Cain's sacrifice fell short of bringing to mind his sin and his need for a Savior. Something in his sacrifice failed to point to the Son of God's work of redemption.
