Marked for Death
Adam and Eve's first two sons differed vastly in their personalities and behavior. Cain longed to farm and build, while Abel loved to roam the hills and meadows with his flocks.
After sin entered this new world because of Adam and Eve's disobedience, God established a sacrificial system and explained that without the shedding of blood there is no forgiveness for sin (Hebrews 9:22).
He told them that these sacrifices pointed forward to the time when Jesus would become a man and die as the Lamb of God for the sins of the world. Abel faithfully brought a young lamb as a sacrifice for his sins, but Cain thought it was unnecessary to obey God's command so precisely. He considered the sacrificial system to be messy and reasoned that as long as he brought an offering and worshiped God, the details wouldn't matter. So he brought an offering of his own works, some produce from the field. Cain watched with jealous anger as fire came down from heaven and consumed Abel's sacrifice but left his own offering untouched. The Lord lovingly urged Cain to humble himself and obey, but Cain stubbornly persisted in his rebellion. Abel also tried to gently reason with his older brother, but Cain flew into a rage. By the time he regained his senses, Abel's bloody body lay still at his feet. God pronounced a curse upon Cain, and when Cain complained about his sentence, the Lord placed a mark upon him lest future generations take revenge for this first homicide.
The book of Revelation tells us that, in the last days, there will again be a battle between Christians regarding how and when to worship. Soon everyone will be identified either by the seal of God or the mark of the beast.