The Geneva Bible Translation Notes, [1599], at sacred-texts.com
(a) Man's nature, the estate of marriage, and God's blessing were not utterly abolished through sin, but the quality or condition of it was changed.
(b) That is, according to the Lord's promise, as some read (Gen 3:15), "To the Lord" rejoicing for the son she had born, whom she would offer to the Lord as the first fruits of her birth.
And in process of time it came to pass, that Cain brought of the fruit of the ground an (c) offering unto the LORD.
(c) This declares that the father instructed his children in the knowledge of God, and also how God gave them sacrifices to signify their salvation, though they were destitute of the ordinance of the tree of life.
If thou doest well, shalt thou not be (e) accepted? and if thou doest not well, sin lieth at the (f) door. And unto thee [shall be] his (g) desire, and thou shalt rule over him.
(e) Both you and your sacrifice shall be acceptable to me.
(f) Sin will still torment your conscience.
(g) The dignity of the first born is given to Cain over Abel.
And the LORD said unto Cain, Where [is] Abel thy brother? And he said, I know not: (h) [Am] I my brother's keeper?
(h) This is the nature of the reprobate when they are rebuke for their hypocrisy, even to neglect God and outrage him.
And he said, What hast thou done? the (i) voice of thy brother's blood crieth unto me from the ground.
(i) God avenges the wrongs against his saints, though no one complains: for the iniquity itself cries for vengeance.
And now [art] thou cursed (k) from the earth, which hath opened her mouth to receive thy brother's blood from thy hand;
(k) The earth will be a witness against you, which mercifully received the blood you most cruelly shed.
When thou tillest the ground, it shall not henceforth yield unto thee her strength; a (l) fugitive and a vagabond shalt thou be in the earth.
(l) You will never have rest for your heart will be in continual fear and worry.
And Cain said unto the LORD, (m) My punishment [is] greater than I can bear.
(m) He burdens God as a cruel judge because he punished him so severely.
And the LORD said unto him, Therefore whosoever slayeth Cain, (n) vengeance shall be taken on him sevenfold. And the LORD set a (o) mark upon Cain, lest any finding him should kill him.
(n) Not for the love he had for Cain, but to suppress murder.
(o) Which was some visible sign of God's judgment, that others should fear by it.
And Cain knew his wife; and she conceived, and bare Enoch: and he builded a (p) city, and called the name of the city, after the name of his son, Enoch.
(p) Thinking by this to be safe, and to have less reason to fear God's judgments against him.
And Lamech took unto him (q) two wives: the name of the one [was] Adah, and the name of the other Zillah.
(q) The lawful institution of marriage, which is, that two should be one flesh, was first corrupted in the house of Cain by Lamech.
And Lamech said unto his wives, Adah and Zillah, Hear my voice; ye wives of Lamech, hearken unto my speech: (r) for I have slain a man to my wounding, and a young man to my hurt.
(r) His wives seeing that all men hated him for his cruelty, were afraid, therefore he brags that there is none strong enough to resist, even though he was already wounded.
If Cain shall be avenged sevenfold, truly Lamech (s) seventy and sevenfold.
(s) He mocked at God's tolerance in Cain jesting as though God would allow no one to punish him and yet give him permission to murder others.
And to Seth, to him also there was born a son; and he called his name Enos: then began men to (t) call upon the name of the LORD.
(t) In these days God began to move the hearts of the godly to restore religion, which had been suppressed by the wicked for a long time.