After the fall of man, the first lesson we are taught is the lesson of Cain and Abel. It is a lesson that shows to us the basic struggle in the world among men, and where it springs from.
“For those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit, the things of the Spirit. For to be carnally minded is death, but to be spiritually minded is life and peace. Because the carnal mind is enmity against God; for it is not subject to the law of God, nor indeed can be. So then, those who are in the flesh cannot please God,” Romans 8:5-8.
Abel was righteous, and Cain was worldly,
“By faith Abel offered to God a more excellent sacrifice than Cain, through which he obtained witness that he was righteous, God testifying of his gifts; and through it he being dead still speaks,” Hebrews 11:4, and,
“For this is the message that you heard from the beginning, that we should love one another, not as Cain who was of the wicked one and murdered his brother. And why did he murder him? Because his works were evil and his brother's righteous,” 1 John 3:11-12.
Abel did not kill Cain; Cain killed Abel. Abel knew that his brother was ungodly, yet did not seek to enslave him, or seek to kill him. Cain knew his brother was righteous, and couldn’t bear it, so he killed him. It is never the one who keeps a contract who squabbles, it is the one who seeks to break it.
After this, we have the history of the offspring of Cain. It is the history of those who pursue after the things of the world, not after God. From Lamech, the great-great-great grandson of Cain, we have the beginnings of polygamy. He took the institution of God, “ Therefore a man shall leave his father and mother and be joined to his wife, and they shall become one flesh,” Genesis 2:24, and pursued sensuality with it. He was a murderer, and declared he would pursue vengeance upon all who hurt him.
The world pursues evil, because it pursues the desires of the flesh, “[A]mong whom also we all once conducted ourselves in the lusts of our flesh, fulfilling the desires of the flesh and of the mind, and were by nature children of wrath, just as the others,” Ephesians 2:3. I don’t mean this as limited to sexual desire, but rather, the sensual things, the things we sense through our body (hot and cold, pain and pleasure, fullness and hunger, etc.). In having a mind focused on the things needed and desired in this world, man forgets the things of God.
It is the same reason that troubles arise in the church,
“Where do wars and fights come from among you? Do they not come from your desires for pleasure that war in your members? You lust and do not have. You murder and covet and cannot obtain. You fight and war. Yet you do not have because you do not ask. You ask and do not receive, because you ask amiss, that you may spend it on your pleasures. Adulterers and adulteresses! Do you not know that friendship with the world is enmity with God? Whoever therefore wants to be a friend of the world makes himself an enemy of God. Or do you think that the Scripture says in vain, ‘The Spirit who dwells in us yearns jealously’?” James 4:1-5.
And so John says, “In this the children of God and the children of the devil are manifest: Whoever does not practice righteousness is not of God, nor is he who does not love his brother. For this is the message that you heard from the beginning, that we should love one another, not as Cain who was of the wicked one and murdered his brother. And why did he murder him? Because his works were evil and his brother's righteous,” 1 John 3:10-12.
And so we are taught,
“By faith Abel offered to God a more excellent sacrifice than Cain, through which he obtained witness that he was righteous, God testifying of his gifts; and through it he being dead still speaks,” Hebrews 11:1-4
“Do not marvel, my brethren, if the world hates you. We know that we have passed from death to life, because we love the brethren. He who does not love his brother abides in death. Whoever hates his brother is a murderer, and you know that no murderer has eternal life abiding in him,” 1 John 3:13-15.