From the Word
10 By this it may be seen who are the children of God, and who are the children of the devil: whoever does not do right is not of God, nor he who does not love his brother.
11 For this is the message which you have heard from the beginning, that we should love one another, 12 and not be like Cain who was of the evil one and murdered his brother. And why did he murder him? Because his own deeds were evil and his brother’s righteous.
1 John 3:10–12, RSV
What offense had godly Abel committed against his brother to be so hated? He had regarded that brother as the first-born, had done him all honor and loved him as became a brother. He was easily satisfied, desiring simply the grace of God. He prayed for the future seed, for the salvation and happiness of his parents, his brother and the entire human race. How could Cain be so unmerciful and inhuman to murder his own flesh and blood?
The answer is found in the fact that the devil had filled Cain’s heart with pride and vanity over his birthright. He considered himself a man of distinction, whilst his brother was nothing. His heart is devoid of true brotherly love. He cannot endure God’s manifest favor toward his brother, and will not be moved by the injunction to humble himself and seek God’s grace. Anger and envy possess him so that he cannot tolerate his brother alive. He becomes a murderer, and then goes his way as if he had done right. This is what John means when he says that Cain had no other cause for his crime than that his works were evil and his brother’s righteous.
Similarly that obedient daughter of Saint Cain, the world, hates the Christians; and for no other reason than the latter’s love and goodness of heart. In this man Cain is pictured the world in its true characteristic colors; in him its true spirit stands reflected. On the other hand that poor, abject Abel well represents the obscure little brotherhood, the Church of Christ. She must yield to Cain, the lord, the distinction of being everything before God. He feels important in his imagined dignity and thinks that God cannot but favor and accept his offering rather than that of his brother.
Meanwhile pious Abel goes his way, meekly suffering his brother’s contempt. He yields him the honor and beholds no consolation for himself aside from the pure mercy and goodness of God. He believes in God and in such faith he performs his sacrifice as a confession of his gratitude.
Luther, Martin, and John Sander. Devotional Readings from Luther’s Works for Every Day of the Year. Augustana Book Concern, 1915, p. 227.