Now Abel kept flocks, and Cain worked the soil. In the course of time Cain brought some of the fruits of the
soil as an offering to the LORD. But Abel brought fat portions from some of the firstborn of his flock.
The LORD looked with favor on Abel and his offering,
but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast.
The New Testament's commentary on this passage offers us some insight. Hebrews 11:4 says, "By faith Abel offered to God a more acceptable sacrifice than Cain, through which he was commended as righteous, God commending him by accepting his gifts." 1 John 3:12 says Cain murdered Abel because Cain's deeds were evil, the deeds in question likely including his sacrifice. These offer further evidence that Cain's offering was wrong in some way, as well as implying that he did not act out of faith.
Why was Cain's offering unacceptable? Some people have suggested that it was because he offered plants while Abel offered animals. I don't think this is necessarily the case, since God accepted grain offerings at other times (Lev. 2) and since the laws regarding sacrifices had not yet been given (though it is interesting to note that Abel offered fat portions as prescribed in the Levitical law - see Lev. 3). It's possible that Cain and Abel had received some instruction on what constituted an acceptable sacrifice from God, or that Cain had committed other wrongs that would make his offering unacceptable to God. However, the passage does give us some clues. It says that Abel "brought fat portions from some of the firstborn." He offered some of what came first, as opposed to waiting until an animal had plenty of offspring (and the oldest were reproducing themselves) and then sacrificing one of the youngest, and he offered the choicest parts. Abel was clearly giving the best of what he had to God. Cain, on the other hand, brought crops, which were not necessarily the best crops - they may have been damaged and/or been what Cain considered "extra" or "leftover." Abel's and Cain's actions were a reflection of their attitudes towards God: should God receive the best of what they had or not?
The question remains, was Cain at fault for not offering the right kind of sacrifice? John R. Cross points out that apparently God talked to Cain and Abel directly, often enough that Cain was accustomed to it and didn't show signs of shock or fall down on his face as others are recorded to have done later in history. I would add that Cain's reply to God in Genesis 4:9 is further evidence of this -- it's the kind of reply a surly teen would make to a parent, which isn't what one would expect if hearing God's voice was a novelty to Cain. Secondly, Cross refers to Genesis 4:7 as God's words don't make sense unless Cain already had an understanding of right and wrong. It therefore makes sense that God had previously given instructions concerning offerings.
Finally, God did not punish Cain until after Cain murdered Abel. Cain was angry when God didn't accept his offering, and God
spoke to him about it and encouraged him to do the right thing so that he would be accepted
(Gen 4:7). This response is hardly unfair or unjust.
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