4:3-4 It is plainly shown that both brothers had faith in God; both either urged by nature or taught by their parents, knew that gifts should be offered to God and that the sin of the paternal transgression should be purged by sacrifices offered to Him. (St. Bede) Therefore there were some ceremonies before the Law, but they were not legal ceremonies, because they were not as yet established by legislation. (St. Thomas Aquinas)

4:5 Cain was not condemned because the kind of offering he made was worthless, for he offered to God from by which he was himself was accustomed to live. But because of the unrighteous mind of the offerer, he was cast down together with his gifts by the one who looks into the heart. (St. Bede) Cain was exceedingly angry: It was not his sin that grieved him, but his brother's success. (Theodoret of Cyrus)

4:8 where is your brother Abel: God put the question with an appearance of uncertainty, in order that even here He might prove man to be the subject of a free will in the alternative of either a denial or a confession, and give to him the opportunity of freely acknowledging his transgression, and, so far, of lightening it. He inquires of Cain where his brother was, just as if He had not yet heard the blood of Abel crying from the ground, in order that he too might have the opportunity from the same power of the will of spontaneously denying, and to this degree aggravating, his crime; and that thus there might be supplied to us examples of confessing sins rather than of denying them. (Tertullian)

4:9 His answer is at once foolish and arrogant; foolish since he supposed that God can be deceived; arrogant, since, he denied that he was his brother’s keeper, when, in his capacity as older brother he ought to have taken care of him. (St. Bede)

4:10 the voice of your brother's blood cries to me: This cry is nothing other than the manifestation of Abel's innocence proclaiming that Cain's wickedness needs to be punished in satisfaction for his death. (Nicholas of Lyra)

4:11-14 A remarkable kind of punishment, and a heavy weight of indignation: You will endure the labor, he is saying, you will make every effort on your part and till the soil stained with such blood, but far from experiencing any reward of your many labors you will find all the exertion you expend will be to no avail. Nor will the effects of punishment stop there, a constant lament and trepidation. In other words, I sentence you to endless shuddering and trembling so that you may have in person not only a constant awareness and reminder of this unholy deed, but also that everyone may see you and learn from this sight. (St. John Chrysostom)

4:15 The sign itself, of course, that he would live, always trembling and groaning, and a vagabond and a fugitive, but reminded by this same affliction of his that he was not permitted to be killed at random by just anyone. For anyone who killed him would not only free Cain from great afflictions but by doing so would subject himself to a sevenfold penalty. (St. Bede)

4:16 east side of Eden: Cain did not live far from the garden, for the reason that by being opposite it he might have a constant reminder also of what had happened to his father because of the Fall and also the magnitude of the crimes committed by him. (St. John Chrysostom)

4:17 Cain knew his wife: Whom did Cain marry? His sister of course. At the time, this was not an offense, no law forbidding it, especially since there was no other way to provide the increase of the race. Therefore, God formed one man from the earth, created one woman for him, and filled the whole world with their offspring. To achieve this goal, He allowed intercourse of brother and sister in the beginning, but when the race had increased, He made this kind of marriage unlawful. Hence, in the ark along with Noah and his sons he saved also the wives of his sons, so that the boys could be married to their cousins. (Theodoret of Cyrus)

4:23-24 Whom did Lamech kill? Not two people as some commentators suppose, nor Cain, a tale invented by others, but one, a young man. The text says, note, “I killed a man as a wound to myself and a youth as a bruise to myself”; that is, a man in his youth. Yet he escaped punishment by confessing the sin, and, by delivering sentence on himself, headed off divine verdict. (Theodoret of Cyrus) Jewish tradition says that Lamech slew a man in mistake for a wild beast. Lamech did not take sufficient care to avoid taking a man's life: and so he was not excused from being guilty of homicide. (St. Thomas Aquinas)