Cain teaches us about love by showing us its opposite. His act of murder so soon after the fall shows us what had entered man’s heart, for Cain is a pattern of the hatred which every unbeliever is capable of feeling towards the believer.
It is necessary for us to know that we are accepted by God before we can love the brethren, for otherwise righteousness becomes a competition, and God’s people become our adversaries.
If Cain is a true example, why is the world not full of murder? Because of the restraining influence of God, and because he ensures a residue of human love even in those who are fallen by nature, for otherwise there would be no language we are capable of understanding by which God could communicate his love towards this lost world.