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.
I think of how each of us today is like Cain, because everybody in the world has done something wrong (Romans 3:23), and all of that sin is a very bad thing and worthy of eternal death (Romans 6:23a). I thought is was intriguing that the Lord bothered to protect Cain, but I think that the Lord had mercy on Cain because Cain called on Him.
This just goes to show that, even when we have done really evil things before, when we admit that we are headed for judgment, and sincerely call on the Lord for mercy, He will save us. The Lord put a mark on Cain to save his life from being killed for his sin, and, in the same way, the Lord puts the mark of Christ on us, seals us with His Holy Spirit, and gives us eternal life, if we call on Him for mercy and have faith in Him.John 6:37
All that the Father giveth me shall come to me; and him that cometh to me I will in no wise cast out.