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Bible students for years have wondered about the mark of Cain. What did God place on him or give him to warn OTHERS not to kill him? The scriptures in question are found in the fourth chapter of Genesis.
The scriptures leading up to the mark of Cain are important to examine in order to get a proper understanding of what God's word does and does not teach. In verse 9 of Genesis 4 God asks him where is his brother Abel. His now well-known response is 'Am I my brother's keeper?' God then confronts him with his grievous sin.
What did God spare the life of Cain instead of taking it? One reason is that he had not yet given man a command not to murder another human. Such a command will be given, however, right after Noah leaves the ark after the floodwaters subside (Genesis 9:6).
An additional (and likely more important) reason he was allowed to live is so that other humans would be warned of God's view and judgment on sin. If he was put to death as a punishment that death would soon be forgotten (Ecclesiastes 8:10) and the lesson of what happened lost. He is cursed, instead, to wander the earth with no permanent home. His constant wandering (due to being a fugitive), coupled with a mark he will soon receive, will be a living testament to others not to repeat the same mistake.
Why would anyone greatly fear someone taking vengeance on them ("anyone who finds me will kill me") if, as some assume, no one else but Adam and Eve where living at the time? The Bible does not state Adam's age when either of his first two sons were born. Seth was born after the death of Abel when Adam was 130 years old. The youngest age at which the first patriarchs (Seth, Enosh, Cainan, and all those to Noah) gave birth to their first son was sixty-five.