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After some time, Cain brought some of the fruit of the ground as an offering to the Lord. Abel also brought some of the firstlings of the flock and of the fat of it. The Lord had regard for Abel and his offering, but for Cain and his offering he had no regard. Cain was very angry, and his face dropped. The Lord said to Cain, "Why are you angry? Why are you downcast? If you do what is good, will you not be accepted? If you do not do  good, sin lies at your door."

For this is the message which you have heard from the beginning, that we should love one another, unlike Cain, who was of the wicked one and killed his brother. Why did he kill him? Because his own works were evil and his brother's righteous.

At the beginning God had respect to the gifts of Abel because he offered with singlemindedness and righteousness, but he had no respect for Cain's offering because his heart was divided with envy and malice, which he cherished against his brother. As God says when reproving his hidden thoughts, 'Though you offer rightly, yet, if you do not divide rightly, have you not sinned? Be at rest';[Gen. 4:7, LXX]; since God is not appeased by sacrifice. (Irenaeus, Against Heresies IV:18:3)

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