Genesis 4: 15: And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.
Cain had the Mark of the Beast; he was born via Eve’s adultery with the Serpent.
The physical mark that God put upon Cain was an enhanced intelligence, for he built Enoch City after he went out from the presence of the LORD (Genesis 4: 16 - 17).
Cain’s mark protected him from the avenger of blood; he had the cunning and shrewdness to be one step ahead of his enemies.