And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.
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Augustine of Hippo
[The pagans say, “The Christians] censure the ceremonies of sacrifice, the victims, incense and the rest, which are used in temple worship. Yet the same ceremonies of sacrifice were originated by themselves or by the god they worship, in primitive times, when a god was assumed to need their offerings of first fruits.” This question is evidently derived from that passage in our Scriptures that tells of Cain making an offering to God of the fruits of the earth and Abel of the firstlings of his flocks. We answer that the conclusion to be drawn from it is that sacrifice is a very ancient custom, because our true and sacred Books warn us that it is not to be offered except to the one true God. But God does not need sacrifices, as is most clearly expressed in the same sacred Books: “I said to the Lord, thou art my God, for thou hast no need of my goods,” because in accepting or refusing or receiving them he is looking only to man’s good. God does not derive any benefit from our worship, but ...
Cain was angry because the offering of his brother had been accepted. Cain became angry on account of the fire that had come down and distinguished between the offerings. His face became gloomy because there was laughter in the eyes of his parents and his sisters when his offering was rejected. They had seen that Cain’s offering had been placed in the midst of the fire and yet the fire did not touch it. .
Abel was very discerning in his choice of offerings, whereas Cain showed no such discernment. Abel selected and offered the choicest of his firstborn and of his fat ones, while Cain either offered young grains or certain fruits that are found at the same time as the young grains. Even if his offering had been smaller than that of his brother, it would have been as acceptable as the offering of his brother, had he not brought it with such carelessness. They made their offerings alternately; one offered a lamb of his flock, the other the fruits of the earth. But because Cain had taken such little regard for the first offering that he offered, God refused to accept it in order to teach Cain how he was to make an offering. .
Consider how the Lord of nature added knowledge to conscience. After all, who brought this to our understanding? It was none other than knowledge associated with conscience. The text says, "He brought an offering of the fruits of the earth to the Lord." He knew and understood that he should offer from his own possessions some produce to God as to his master. not because God needs them, but for the purpose of demonstrating his gratitude as being himself a beneficiary of such kindness. God, you see, is proof against need, and depends on nothing we have to offer; but in his ineffable love he shows considerateness for us, and for the sake of our salvation he allows these things to happen so that knowledge of the Lord may be for the human race a school of virtue.