Abel's sacrifice was accepted by God because it was made according to God's instructions. Cain, however, tried to approach God in his own way and was rejected by God (Genesis 4:3-5). From that point on, Cain became a symbol of those who try to approach God on their own terms.
There are very few theorems in advanced analysis which have been demonstrated in a logically tenable manner. Everywhere one finds this miserable way of concluding from the special to the general and it is extremely peculiar that such a procedure has led to so few of the so-called paradoxes.
The mark of Cain is stamped upon our foreheads. Across the centuries, our brother Abel was lain in blood which we drew, and shed tears we caused by forgetting Thy love. Forgive us, Lord, for the curse we falsely attributed to their name as Jews. Forgive us for crucifying Thee a second time in their flesh. For we knew not what we did.
The gods were bored and so they created man. Adam was bored because he was alone, so Eve was created. Thus boredom entered the world, and increased in proportion to the increase in population. Adam was bored alone, then Adam and Eve were bored together; them Adam and Eve and Cain and Abel were bored en famille; then the population of the world increased, and the people were bored en masse.
What always strikes me in the story of Cain and Abel is how often the word "brother" is used. Cain killed his "brother." God says it was "the blood of your brother." The killing was done to another human being, a child of God like you, breaking that sacred bond of common humanity.
I challenge anyone to tell me that there isn’t evil in this world. From the days of Cain and Abel we know all too well there will always be evil. But that evil shouldn’t take away our freedoms. In fact, the only way to defeat evil is by taking advantage of our freedoms.