It was lingering animosity that finally got the best of Cain when he murdered his brother Abel. But, if we read Jesus' words carefully, we will find that Cain's downfall was a bitterness toward his brother which had taken root long before he committed his heinous act. "So if you are offering your gift at the altar and there remember that your brother has something against you," Jesus says, "leave your gift there before the altar and go. First be reconciled to your brother, and then come and offer your gift."
Had Cain been in a right relationship with his brother when he offered his gift, God would have received it. But because Cain was not in a right relationship with is brother he was, likewise, not in a right relationship with God. Bitterness bred jealousy, and jealousy manifest itself in Cain's murderous rage. He had offered his gift from a divided and unreconciled heart, thus allowing sin to gain a foothold in his life.
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