If thou does well. In these words God reproves Cain for having been unjustly angry, inasmuch as the blame of the whole evil lay with himself. For foolish indeed was his complaint and indignation at the rejection of sacrifices, the defects of which he had taken no care to amend. Thus all wicked men, after they have been long and vehemently enraged against God, are at length so convicted by the Divine judgment, that they vainly desire to transfer to others the cause of the evil....
Some of the Hebrew Doctors refer it to the countenance of Cain, as if God promised that he would lift it up though now cast down with sorrow. Other of the Hebrews apply it to the remission of sins; as if it had been said, ‘Do well, and thou shalt obtain pardon’. But because they imagine a satisfaction, which derogates from free pardon, they dissent widely from the meaning of Moses. A third exposition approaches more nearly to the truth, that exaltation is to be taken for honor, in this way, ‘There is no need to envy thy brother’s honor, because, if thou conductest thyself rightly, God will also raise thee to the same degree of honor; though he now, offended by thy sins, has condemned thee to ignominy.’...
We now perceive how unjustly Cain was angry that his sacrifices were not honored seeing that God was ready to receive them with outstretched hands, provided they ceased to be faulty. At the same time, however; what I before said must be recalled to memory, that the chief point of well-doing is, for pious persons, relying on Christ the Mediator, and on the gratuitous reconciliation procured by him, to endeavor to worship God sincerely and without dissimulation. Therefore, these two things are joined together by a mutual connection: that the faithful, as often as they enter into the presence of God, are commended by the grace of Christ alone, their sins being blotted out; and yet that they bring thither true purity of heart.
And if thou does not well. On the other hand, God pronounces a dreadful sentence against Cain, if he harden his mill in wickedness and indulge himself in his crime; for the address is very emphatical, because God not only repels his unjust complaint, but shows that Cain could have no greater adversary than that sin of his which he inwardly cherished. He so binds the impious man, by a few concise words, that he can find no refuge, as if he had said, ‘Thy obstinacy shall not profit thee; for, though thou shouldst have nothing to do with me, thy sin shall give thee no rest, but shall drive thee on, pursue thee, and urge thee, and never suffer thee to escape.’ Hence it follows, that he not only raged in vain and to no profit; but was held guilty by his own inward conviction, even though no one should accuse him; for the expression, ‘Sin lieth at the door’, relates to the interior judgement of the conscience, which presses upon the man convinced of his sin, and besieges him on every side....
And unto thee shall be his desire. Nearly all commentators refer this to sin, and think that, by this admonition, those depraved hosts are restrained which solicit and impel the mind of man. Therefore, according to their view, the meaning will be of this kind, ‘If sin rises against thee to subdue thee, why dost thou indulge it, and not rather labor to restrain and control it? For it is thy part to subdue and bring into obedience those affections in thy flesh which thou perceivest to be opposed to the will of God, and rebellious against him.’ But I suppose that Moses means something entirely different. I omit to notice that to the Hebrew word for sin is affixed the mark of the feminine gender, but that here two masculine relative pronouns are used. Certainly Moses does not treat particularly of the sin itself which was committed, but of the guilt which is contracted from it, and of the consequent condemnation. How, then, do these words suit, ‘Unto thee shall be his desire?’.
There will, however be no need for long refutation when I shall produce the genuine meaning of the expression. It rather seems to be a reproof, by which God charges the impious man with ingratitude, because he held in contempt the honor of primogeniture. The greater are the divine benefits
with which any one of us is adorned, the more does he betray his impiety unless he endeavors earnestly to serve the Author of grace to whom he is under obligation. When Abel was regarded as his brother’s inferior, he was, nevertheless, a diligent worshipper of God. But the firstborn worshipped God negligently and perfunctorily, though he had, by the Divine kindness, arrived at so high a dignity; and, therefore, God enlarges upon his sin, because he had not at least imitated his brother, whom he ought to have surpassed as far in piety as he did in the degree of honor.
Moreover, this form of speech is common among the Hebrews, that the desire of the inferior should be towards him to whose will he is subject; thus Moses speaks of the woman, (Genesis 3:16,) that her desire should be to her husband. They, however, childishly trifle, who distort this passage to prove the freedom of the will; for if we grant that Cain was admonished of his duty in order that he might apply himself to the subjugation of sin, yet no inherent power of man is to be hence inferred; because it is certain that only by the grace of the Holy Spirit can the affections of the flesh be so mortified that they shall not prevail. Nor, truly, must we conclude, that as often as God commands anything we shall have strength to perform it, but rather we must hold fast the saying of Augustine, ‘Give what thou commandest, and command what thou wilt.’