"And the Lord put a mark on Cain, lest any who found him should attack him." Genesis 4:15 ESV
What is the mark placed on Cain?
It seems to have been a mark of warning, not of protection, to those who may think to harm him. The mark was a sign of God's mercy for this murderer. Unfortunately, others warped the meaning of the sign and applied it to themselves later in the same chapter:
Mankind misconstrued God's mark of mercy to be a mark of license, and had no problem killing others who bothered them. To them it was a mark of "revenge" not mercy. Thus, mankind grew worse and worse, since there was no fear of God in them. So the Flood was necessary, and after the Flood, came God's clear command so we would not make the same mistake:
They did not seem to see the mark as a physical mark; it appears that they just applied it to themselves as a matter of principle: "I killed a man, so I am marked by God, too."
The Bible doesn't say what the mark was. I think the verse you quote is all that we're told about it. It is puzzling. I always picture some symbol on his forehead, like a tatoo. But nothing in the text spells that out.
The whole point is that people who knew Cain was a murderer might take it upon themselves to execute him. But that would require that they recognize him and know what he did. So if God had commanded that Cain not be killed, why would an additional mark be necessary? And if it was some sort of symbol, how would anyone know what it means? Or was it words, like "Don't dare kill this man, signed, God"?
A teacher years ago suggested that maybe it wasn't anything like a symbol or tatoo at all, but rather, a look in his eyes. It sounds to me like that could make a great ending to a story. "And then he turned and looked at me, and I knew I couldn't kill him. I dropped my weapon. For suddenly I knew who he was. That haunting, hunted, hated look in his eyes. He bore the mark of Cain." I'd write that story except I can't think of anything interesting to go before that. :-)
Cain and Abel reprsent the two classes of people at the end of time. Cain represents the wicked, and Able represents the righteous.
1 John 3;12 Not as Cain, who was of the wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother's righteous. Matthew 13;19 When anyone heareth the word of the kingdom, and understandeth it not, then cometh the wicked one, and catheth away that which was sown in his heart. This is he which received seed by the way side. 1 John 3;8 He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
Exodus 33;16,17 For wherein shall it be known here that, I and thy people found grace in thy sight? is it not in that thou goest with us? so shall we be seperated, I and thy people, from all the people that are upon the face of the earth. And the LORD said unto Moses, I will do this thing, also that thou has spoken: for thou has found grace in my sight, and I know thee by name.
Isaiah 13;9,11 Behold, the day of the LORD cometh, cruel both with wrath and fierce anger, to lay the land desolate: and he shall destroy the sinners thereof out of it. And I will punish the world for their evil, and the wicked for their iniquity; and I will cause the arrogancy of the proud to cease, and will lay low the haughtiness of the terrible.
Revelation 22;11 He that is unjust, let him be unjust still: and he which is filthy, let him be filthy still: and he that is righteous, let him be righteous still: and he that is holy, let him be holy still.
The only people who survived the flood were Noah, his wife, their 3 sons and their wives, 8 people (Genesis 7:13, 1 Peter 3:20). None of them were descendants of Cain, so the mark of Cain has died out, whatever it was. (Some believe it was the Neanderthals.) Many people, however, confuse the mark of Cain with the curse of Canaan in Genesis 9:25 "Cursed be Canaan; a servant of servants shall he be unto his brethren." That's a completely different thing. The descendants of Canaan were the Canaanites, as you read in Genesis 10, and also the cities of Sodom and Gomorrah were populated by them. So the curse of Canaan sets up the story for the destruction of Sodom and Gomorrah as well as the Israelite conquest of the land of Canaan.