Genesis 4 details the first murder when Cain kills his brother Abel in a fit of angry jealousy. Both brothers bring the fruit of their work as offerings to God. Cain is a farmer, and he brings some of the fruit of the ground, with no indication in the biblical text that this is the first or the best of his produce.
Abel is a shepherd and brings the "firstlings," the best, the “fat portions” of his flock. Although both are producing food, they are neither working nor worshiping together. Work is no longer a place of good relationships. Remember, part of the curse was you would have to work and sweat to survive. That is what they were doing.
But unto Cain and to his offering he had not respect
Not because of the matter of it, as some have thought; but because it was not offered in faith and sincerity, but in a formal and hypocritical manner, without any regard to the Messiah and his sacrifice, and without any view to the glory of God: no notice was taken, no thought or preparation or any other such thing. It was just brought by Cain and “dropped off.
This is why God was not pleased with his offering.
And it is why Cain got mad at his brother - and killed him.
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