2. You have asked for a solution of that famous passage which is everywhere interpreted in different senses, "Whosoever slayeth Cain will exact vengeance for seven sins."  Your question shews that you have yourself carefully observed the charge of Paul to Timothy,  for you are obviously attentive to your reading. You have moreover roused me, old man that I am, dull alike from age and bodily infirmity, and from the many afflictions which have been stirred up round about me and have weighed down my life. Fervent in spirit as you are yourself, you are rousing me, now benumbed like a beast in his den, to some little wakefulness and vital energy. The passage in question may be interpreted simply and may also receive an elaborate explanation. The simpler, and one that may occur to any one off hand, is this: that Cain ought to suffer sevenfold punishment for his sins.
For it is not the part of a righteous judge to define requital on the principle of like for like, but the originator of evil must pay his debt with addition, if he is to be made better by punishment and render other men wiser by his example. Therefore, since it is ordained that Cain pay the penalty of his sin sevenfold, he who kills him, it is said, will discharge the sentence pronounced against him by the divine judgment. This is the sense that suggests itself to us on our first reading the passage.
3. But readers, gifted with greater curiosity, are naturally inclined to probe into the question further. How, they ask, can justice be satisfied seven times? And what are the vengeances? Are they for seven sins committed? Or is the sin committed once and are there seven punishments for the one sin? Scripture continually assigns seven as the number of the remission of sins. "How often," it is asked, "shall my brother sin against me and I forgive him?" (It is Peter who is speaking to the Lord.) "Till seven times?" Then comes the Lord's answer, "I say not unto thee, until seven times, but, until seventy times seven."  Our Lord did not vary the number, but multiplied the seven, and so fixed the limit of the forgiveness. After seven years the Hebrew used to be freed from slavery.  Seven weeks of years used in old times to make the famous jubilee,  in which the land rested, debts were remitted, slaves were set free, and, as it were, a new life began over again, the old life from age to age being in a sense completed at the number seven. These things are types of this present life, which revolves in seven days and passes by, wherein punishments of slighter sins are inflicted, according to the loving care of our good Lord, to save us from being delivered to punishment in the age that has no end. The expression seven times is therefore introduced because of its connexion with this present world for men who love this world ought specially to be punished in the things for the sake of which they have chosen to live wicked lives. If you understand the vengeances to be for the sins committed by Cain, you will find those sins to be seven. Or if you understand them to mean the sentence passed on him by the Judge, you will not go far wrong. To take the crimes of Cain: the first sin is envy at the preference of Abel; the second is guile, whereby he said to his brother, "Let us go into the field:"  the third is murder, a further wickedness: the fourth, fratricide, a still greater iniquity: the fifth that he committed the first murder, and set a bad example to mankind: the sixth wrong in that he grieved his parents: the seventh, his lie to God; for when he was asked, "Where is Abel thy brother?" he replied, "I know not."  Seven sins were therefore avenged in the destruction of Cain. For when the Lord said, "Cursed is the earth which has opened to receive the blood of thy brother," and "groaning and trembling shall there be on the earth," Cain said, "If thou castest me out to-day from the earth, then from thy face shall I be hid, and groaning and trembling shall I lie upon the earth, and every one that findeth me shall slay me." It is in answer to this that the Lord says, "Whosoever slayeth Cain will discharge seven vengeances."  Cain supposed that he would be an easy prey to every one, because of there being no safety for him in the earth (for the earth was cursed for his sake), and of his being deprived of the succour of God, Who was angry with him for the murder, and so of there being no help for him either from earth or from heaven. Therefore he said, "It shall come to pass that every one that findeth me shall slay me." Scripture proves his error in the words, "Not so;" i.e. thou shalt not be slain. For to men suffering punishment, death is a gain, because it brings relief from their pain. But thy life shall be prolonged, that thy punishment may be made commensurate with thy sins. Since then the word ekdikoumenon may be understood in two senses; both the sin for which vengeance was taken, and the manner of the punishment, let us now examine whether the criminal suffered a sevenfold torment.
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 thy 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 thy brother's blood crieth unto me." So the enquiry, "Where is Abel thy 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 thy sake," one punishment. "Thou shalt 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. "Thou shalt till the ground." Terrible punishment, to live with those that hate one, to have for a companion an enemy, an implacable foe. "Thou shalt till the earth," that is, Thou shalt toil at the labours of the field, never resting, never released from thy 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 thee 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 shalt thou 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, "Thou hast driven me out from the face of the earth, and from thy 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 thy 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 findeth me shall slay me." He infers this from what has gone before. If I am cast out of the earth, and hidden from thy 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." 
5. Your next question is of a kindred character, concerning the words of Lamech to his wives; "I have slain a man to my wounding, and a young man to my hurt: if Cain shall be avenged sevenfold, truly Lamech seventy and sevenfold."  Some suppose that Cain was slain by Lamech, and that he survived to this generation that he might suffer a longer punishment. But this is not the case. Lamech evidently committed two murders, from what he says himself, "I have slain a man and a young man," the man to his wounding, and the young man to his hurt. There is a difference between wounding and hurt.  And there is a difference between a man and a young man. "If Cain shall be avenged sevenfold, truly Lamech seventy and sevenfold." It is right that I should undergo four hundred and ninety punishments, if God's judgment on Cain was just, that his punishments should be seven. Cain had not learned to murder from another, and had never seen a murderer undergoing punishment. But I, who had before my eyes Cain groaning and trembling, and the mightiness of the wrath of God, was not made wiser by the example before me. Wherefore I deserve to suffer four hundred and ninety punishments. There are, however, some who have gone so far as the following explanation, which does not jar with the doctrine of the Church; from Cain to the flood, they say, seven generations passed by, and the punishment was brought on the whole earth, because sin was everywhere spread abroad. But the sin of Lamech requires for its cure not a Flood, but Him Who Himself takes away the sin of the world.  Count the generations from Adam to the coming of Christ, and you will find, according to the genealogy of Luke, that the Lord was born in the seventy-seventh.
Thus I have investigated this point to the best of my ability, though I have passed by matters therein that might be investigated, for fear of prolonging my observations beyond the limits of my letter. But for your intelligence little seeds are enough. "Give instruction," it is said, "to a wise man, and he will be yet wiser."  ^ "If a skilful man hear a wise word he will commend it, and add unto it." 
6. About the words of Simeon to Mary, there is no obscurity or variety of interpretation. "And Simeon blessed them, and said unto Mary His mother, Behold, this Child is set for the fall and rising again of many in Israel; and for a sign which shall be spoken against; (yea, a sword shall pierce through thine own soul also,) that the thoughts of many hearts may be revealed."  Here I am astonished that, after passing by the previous words as requiring no explanation, you should enquire about the expression, "Yea, a sword shall pierce through thy own soul also." To me the question, how the same child can be for the fall and rising again, and what is the sign that shall be spoken against, does not seem less perplexing than the question how a sword shall pierce through Mary's heart.
7. My view is, that the Lord is for falling and rising again, not because some fall and others rise again, but because in us the worst falls and the better is set up. The advent  of the Lord is destructive of our bodily affections and it rouses the proper qualities of the soul. As when Paul says, "When I am weak, then I am strong,"  the same man is weak and is strong, but he is weak in the flesh and strong in the spirit. Thus the Lord does not give to some occasions of falling and to others occasions of rising. Those who fall, fall from the station in which they once were, but it is plain that the faithless man never stands, but is always dragged along the ground with the serpent whom he follows. He has then nowhere to fall from, because he has already been cast down by his unbelief. Wherefore the first boon is, that he who stands in his sin should fall and die, and then should live in righteousness and rise, both of which graces our faith in Christ confers on us. Let the worse fall that the better may have opportunity to rise. If fornication fall not, chastity does not rise. Unless our unreason be crushed our reason will not come to perfection. In this sense he is for the fall and rising again of many.
8. For a sign that shall be spoken against. By a sign, we properly understand in Scripture a cross. Moses, it is said, set the serpent "upon a pole."  That is upon a cross. Or else a sign  is indicative of something strange and obscure seen by the simple but understood by the intelligent. There is no cessation of controversy about the Incarnation of the Lord; some asserting that he assumed a body, and others that his sojourn was bodiless; some that he had a passible body, and others that he fulfilled the bodily oeconomy by a kind of appearance. Some say that his body was earthly, some that it was heavenly; some that He pre-existed before the ages; some that He took His beginning from Mary. It is on this account that He is a sign that shall be spoken against.
9. By a sword is meant the word which tries and judges our thoughts, which pierces even to the dividing asunder of soul and spirit and of the joints and marrow, and is a discerner of our thoughts.  Now every soul in the hour of the Passion was subjected, as it were, to a kind of searching. According to the word of the Lord it is said, "All ye shall be offended because of me."  Simeon therefore prophesies about Mary herself, that when standing by the cross, and beholding what is being done, and hearing the voices, after the witness of Gabriel, after her secret knowledge of the divine conception, after the great exhibition of miracles, she shall feel about her soul a mighty tempest.  The Lord was bound to taste of death for every man -- to become a propitiation for the world and to justify all men by His own blood. Even thou thyself, who hast been taught from on high the things concerning the Lord, shalt be reached by some doubt. This is the sword. "That the thoughts of many hearts may be revealed." He indicates that after the offence at the Cross of Christ a certain swift healing shall come from the Lord to the disciples and to Mary herself, confirming their heart in faith in Him. In the same way we saw Peter, after he had been offended, holding more firmly to his faith in Christ. What was human in him was proved unsound, that the power of the Lord might be shewn.
 Placed in 377.  Bishop of Antioch in Pisidia. Soc. vii. 36; Theod. v. 8.  Genesis 4:15, LXX.  cf. 1 Tim. iv. 13.  Matthew 18:21, 22.  Deuteronomy 5:12.  Leviticus 25:10.  Genesis 4:8.  Genesis 4:9.  Genesis 4:11, 12, 14, 15, LXX.  Daniel 12:2.  Genesis 4:23, 24.  LXX. molops, i.e. weal.  John 1:29.  Proverbs 9:9.  Ecclus. xx. 18.  Luke 2:34, 35.  epidaneia.  2 Corinthians 12:10.  Numbers 21:8.  semeion, LXX.  cf. Hebrews 4:12.  Matthew 26:3.  The Ben. note strongly objects to this slur upon the constancy of the faith of the Blessed Virgin, and is sure that St. Basil's error will not be thus corrected without his own concurrence. It supposes this interpretation of the passage in question to be derived from Origen, Hom. xxvii. In Lucam, and refers to a list of commentators who have followed him in Petavius, De Incar. xiv. 1.
 Bishop of Antioch in Pisidia. Soc. vii. 36; Theod. v. 8.
 Genesis 4:15, LXX.
 cf. 1 Tim. iv. 13.
 Matthew 18:21, 22.
 Deuteronomy 5:12.
 Leviticus 25:10.
 Genesis 4:8.
 Genesis 4:9.
 Genesis 4:11, 12, 14, 15, LXX.
 Daniel 12:2.
 Genesis 4:23, 24.
 LXX. molops, i.e. weal.
 John 1:29.
 Proverbs 9:9.
 Ecclus. xx. 18.
 Luke 2:34, 35.
 2 Corinthians 12:10.
 Numbers 21:8.
 semeion, LXX.
 cf. Hebrews 4:12.
 Matthew 26:3.
 The Ben. note strongly objects to this slur upon the constancy of the faith of the Blessed Virgin, and is sure that St. Basil's error will not be thus corrected without his own concurrence. It supposes this interpretation of the passage in question to be derived from Origen, Hom. xxvii. In Lucam, and refers to a list of commentators who have followed him in Petavius, De Incar. xiv. 1.