DID YOU KNOW?
The answer
To protect him. Cain has just said, "anyone who meets me may kill me" (Genesis 4:14). God answers, "Not so!" and puts a mark on Cain "so that no one who came upon him would kill him" (Genesis 4:15). The text never says what the mark looked like, and never calls it a punishment. Cain's punishment comes three verses earlier: the ground will no longer yield to him, and he will be a wanderer (Genesis 4:12).
Why it’s interesting
Guesses about what the mark looked like have gone on for centuries. Whatever it was, it did its job: Cain lives on to build the Bible's first city (Genesis 4:17). The story that opens with a murder closes with the murderer under God's protection.
Scholarly consensus
Learn the Bible.
Not just the stories, but the history, authorship, and scholarship behind it.