4 The seven sins of Cain have been enumerated in what has been already said. Now I ask if the punishments inflicted on him were seven, and I state as follows. The Lord enquired 'Where is Abel your brother?' not because he wished for information, but in order to give Cain an opportunity for repentance, as is proved by the words themselves, for on his denial the Lord immediately convicts him saying, “The voice of your brother's blood cries unto me.” So the enquiry, “Where is Abel your brother?” was not made with a view to God's information, but to give Cain an opportunity of perceiving his sin.
But for God's having visited him he might have pleaded that he was left alone and had no opportunity given him for repentance. Now the physician appeared that the patient might flee to him for help. Cain, however, not only fails to hide his sore, but makes another one in adding the lie to the murder. “I know not. Am I my brother's keeper?” Now from this point begin to reckon the punishments. “Cursed is the ground for your sake,” one punishment. “You shall till the ground.” This is the second punishment.
Some secret necessity was imposed upon him forcing him to the tillage of the earth, so that it should never be permitted him to take rest when he might wish, but ever to suffer pain with the earth, his enemy, which, by polluting it with his brother's blood, he had made accursed. “You shall till the ground.” Terrible punishment, to live with those that hate one, to have for a companion an enemy, an implacable foe. “You shall till the earth,” that is, You shall toil at the labours of the field, never resting, never released from your work, day or night, bound down by secret necessity which is harder than any savage master, and continually urged on to labour.
“And it shall not yield unto you her strength.” Although the ceaseless toil had some fruit, the labour itself were no little torture to one forced never to relax it. But the toil is ceaseless, and the labours at the earth are fruitless (for “she did not yield her strength”) and this fruitlessness of labour is the third punishment. “Groaning and trembling shall you be on the earth.” Here two more are added to the three; continual groaning, and tremblings of the body, the limbs being deprived of the steadiness that comes of strength.
Cain had made a bad use of the strength of his body, and so its vigour was destroyed, and it tottered and shook, and it was hard for him to lift meat and drink to his mouth, for after his impious conduct, his wicked hand was no longer allowed to minister to his body's needs. Another punishment is that which Cain disclosed when he said, “You have driven me out from the face of the earth, and from your face shall I be hid.” What is the meaning of this driving out from the face of the earth?
It means deprivation of the benefits which are derived from the earth. He was not transferred to another place, but he was made a stranger to all the good things of earth. “And from your face shall I be hid.” The heaviest punishment for men of good heart is alienation from God. “And it shall come to pass that every one that finds me shall slay me.” He infers this from what has gone before. If I am cast out of the earth, and hidden from your face, it remains for me to be slain of every one.
What says the Lord? Not so. But he put a mark upon him. This is the seventh punishment, that the punishment should not be hid, but that by a plain sign proclamation should be made to all, that this is the first doer of unholy deeds. To all who reason rightly the heaviest of punishments is shame. We have learned this also in the case of the judgments, when “some” shall rise “to everlasting life, and some to shame and everlasting contempt.”
Source: Letters (New Advent)