According to Genesis 5:4, Adam and Eve had sons and daughters. Thus, Cain doubtless married one of his sisters. This verse also explains what Cain was afraid of after he had murdered his brother in Genesis 4:14. He no doubt assumed his parents would bear other sons and daughters and that one of them might someday come looking for him.

Perhaps no other question concerning the Bible has been asked more than this one. However, to say the least, it is absolutely insignificant as compared to the one asked by a Philippian jailer, “What must I do to be saved?” (Acts 16:30).