Genesis 4:11

You see, since Cain perpetrated practically the same evil as the serpent, which like an instrument served the devil's purposes, and as the serpent introduced mortality by means of deceit, in like manner Cain deceived his brother, led him out into open country, raised his hand in armed assault against him and committed murder. Hence, as God said to the serpent, "Cursed are you beyond all the wild animals of the earth," so to Cain too when he committed the same evil as the serpent.

Now therefore you are cursed from the ground, which has opened its mouth to receive your brother's blood from your hand. He refers to the earth as the Church, for no other earth but she opens her mouth to taste in the mystery the blood of Christ that was shed by the hands of the Jewish people. Over this Church, the same people are accursed: for as much as she more closely adheres to the love of her Creator, so much more gravely does she understand the nation opposed to Him to be cursed, even if they, still in their pride, boast themselves as specially blessed.

Now therefore you are cursed from the earth which has opened its mouth to receive your brother's blood from your hand: when you work the ground, it shall no longer yield to you its strength; you shall be a fugitive and a wanderer on the earth. According to the measure of sinners, the just judge also sets the measure of retribution: for Adam, who ate the forbidden fruit, is by no means himself cursed, but rather the earth is subjected to a curse in his work; not so much that it denies the fruits to the worker, but that it yields its fruits to the cultivator with labor and sweat; and so that the same labor could eventually be finished, and eternal rest entered. He is also subjected to the death of the flesh: which is done by a favor of benevolence, so that he would not always labor and suffer, as only those who cannot grasp how burdensome the life of this present age is would doubt. But Cain, though knowing the transgression of his parents and their condemnation, added to the same transgression a heavier guilt of perfidy, malice, murder, and lying, which he had inherited; hence he is also justly punished with a harsher penalty: first, that he himself should be cursed beyond the earth, which he had polluted with his brother's blood, previously clean; then, that he should sweat uselessly in working the earth, with no abundance of fruits responding to his labors: thirdly, that he should always be a wanderer and a fugitive on the earth, not daring to have settled abodes anywhere, or, as another translation has it, live trembling and lamenting on the earth. But he, having heard the penalty of his great curse, did not wish to ask for forgiveness, but doubling sins upon sins, thought his crime so great that it could not be forgiven by God. Finally, he responded to the Lord: