In Genesis 4:17 we read that “Cain knew his wife, and she conceived and bore Enoch.” Many people wonder where Cain found a wife, and this question undermines their faith in the book of Genesis. Actually, though, the question is not difficult to answer.

Adam and Eve had other children after the births of Cain, Abel, and Seth. Genesis 5:4 tells us, “The days of Adam after he fathered Seth were 800 years; and he had other sons and daughters.” Eight hundred years is a long time to be fruitful and multiply (Genesis 1:28)!

Adam and Eve were the first man and woman, and God had commanded them and their descendants to be fruitful and multiply, so Cain probably married one of his many sisters. Given the long life spans at that time, he could have married a niece or even a grandniece.

In the early years of the human race, no genetic defects had yet developed as a result of the fall of man. By the time of Abraham, God had not yet declared this kind of marriage to be contrary to His will (see Genesis 20:12). Apparently, incest was not prohibited until the time of Moses (Leviticus 18:7-17; 20:11-12,14,17,20-21).

Ron Rhodes, 5-Minute Apologetics for Today: 365 Quick Answers to Key Questions (Eugene, OR: Harvest House Publishers, 2010).