Of the Cause of Cain's Crime and His Obstinacy, Which Not Even the Word of God Could Subdue
Yet He does not dismiss him without counsel, holy, just, and good. “Fret not yourself,” He says, “for unto you shall be his turning, and you shall rule over him.” Over his brother, does He mean? Most certainly not. Over what, then, but sin? For He had said, “You have sinned,” and then He added, “Fret not yourself, for to you shall be its turning, and you shall rule over it.” And the “turning” of sin to the man can be understood of his conviction that the guilt of sin can be laid at no other man's door but his own.
For this is the health-giving medicine of penitence, and the fit plea for pardon; so that, when it is said, “To you its turning,” we must not supply “shall be,” but we must read, “To you let its turning be,” understanding it as a command, not as a prediction. For then shall a man rule over his sin when he does not prefer it to himself and defend it, but subjects it by repentance; otherwise he that becomes protector of it shall surely become its prisoner. But if we understand this sin to be that carnal concupiscence of which the apostle says, “The flesh lusts against the spirit,” among the fruits of which lust he names envy, by which assuredly Cain was stung and excited to destroy his brother, then we may properly supply the words “shall be,” and read, “To you shall be its turning, and you shall rule over it.”
For when the carnal part which the apostle calls sin, in that place where he says, “It is not I who do it, but sin that dwells in me,” that part which the philosophers also call vicious, and which ought not to lead the mind, but which the mind ought to rule and restrain by reason from illicit motions—when, then, this part has been moved to perpetrate any wickedness, if it be curbed and if it obey the word of the apostle, “Yield not your members instruments of unrighteousness unto sin,” it is turned towards the mind and subdued and conquered by it, so that reason rules over it as a subject.
It was this which God enjoined on him who was kindled with the fire of envy against his brother, so that he sought to put out of the way him whom he should have set as an example. “Fret not yourself,” or compose yourself, He says: withhold your hand from crime; let not sin reign in your mortal body to fulfill it in the lusts thereof, nor yield your members instruments of unrighteousness unto sin. “For to you shall be its turning,” so long as you do not encourage it by giving it the rein, but bridle it by quenching its fire.
“And you shall rule over it;” for when it is not allowed any external actings, it yields itself to the rule of the governing mind and righteous will, and ceases from even internal motions. There is something similar said in the same divine book of the woman, when God questioned and judged them after their sin, and pronounced sentence on them all—the devil in the form of the serpent, the woman and her husband in their own persons. For when He had said to her, “I will greatly multiply your sorrow and your conception; in sorrow shall you bring forth children,” then He added, “and your turning shall be to your husband, and he shall rule over you.” What is said to Cain about his sin, or about the vicious concupiscence of his flesh, is here said of the woman who had sinned; and we are to understand that the husband is to rule his wife as the soul rules the flesh.
And therefore, says the apostle, “He that loves his wife, loves himself; for no man ever yet hated his own flesh.” This flesh, then, is to be healed, because it belongs to ourselves: is not to be abandoned to destruction as if it were alien to our nature. But Cain received that counsel of God in the spirit of one who did not wish to amend. In fact, the vice of envy grew stronger in him; and, having entrapped his brother, he slew him. Such was the founder of the earthly city. He was also a figure of the Jews who slew Christ the Shepherd of the flock of men, prefigured by Abel the shepherd of sheep: but as this is an allegorical and prophetical matter, I forbear to explain it now; besides, I remember that I have made some remarks upon it in writing against Faustus the Manichæan.
Source: City of God (New Advent)