Where did Cain's wife come from? Who was she?

from the old testament

• When God saw His creation, He said it was "very good" (Genesis 1:31). Adam and Eve had no genetic defects.
• When Adam and Eve sinned, they were expelled from the garden of Eden, and the earth was cursed (Genesis 3:14–24). Sin brought death, and people began experiencing the effects of the fall, including effects on genetics. In Cain’s time, however, there was likely no genetic problem with marrying a close relative.
• There was no way to follow God’s call to “be fruitful and multiply and fill the earth and subdue it” (Genesis 1:28) without marrying relatives both in the beginning with Adam and Eve and after the flood with Noah’s family (Genesis 9:1, 7, 18–19).
• Although God had previously allowed incest, in the time of Moses, God gave the nation of Israel specific laws regarding marriage with close relations (Leviticus 18; 20) and specifically on sexual relations to close relatives (Leviticus 18:6).
• God wanted the Israelites to be separate and live differently than the other nations, so they would know that He is the Lord (i.e., Leviticus 18:6, 30; 20:8). Other nations intermarried. For example, the Egyptian Pharaohs were so protective of their bloodline that marriages were generally only between close relatives.
• The relationship laws God created for Israel revealed the value He places on the family as well as its individual members just as much as genetic concerns. God’s laws are for our good and His glory. Jacob certainly learned that marrying two sisters made for a less than peaceful household (see Genesis 29—30).
• Just because Cain, Abel, and Seth are the only children of Adam and Eve mentioned by name (Genesis 4:1, 2, 25) does not mean they were the only ones in existence. Genesis 5:4 tells us that Adam and Eve had many children, meaning by the time Cain married, he would have had several options among his relatives.

from the new testament

• The New Testament does not mention Cain’s wife.
• The New Testament confirms that all people of the world came from the eight people who trusted God and went into the ark during the flood (1 Peter 3:20). These eight people came from Adam and Eve.
• The New Testament upholds the same standards for sexual relations as the Old Testament (Matthew 19:4–5; 1 Corinthians 5:1; 6:18).

implications for today

Incest has been prohibited since the time of Moses. God used the Mosaic law to reveal how He called the nation of Israel to live so they could reflect Him to the world. God created marriage and has boundaries around what He seeks to protect (Genesis 2:24). Although we do not live according to the commandments God gave Israel, they reveal to us what God is like, what is important to Him, and principles for living it out. Marriage and sexual purity are still equally as important to God today (Hebrews 13:4).

First Corinthians 6:18 calls us to, “Flee from sexual immorality. Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body.” Sexual sin has consequences that devastate us and others. Sin brings death (James 1:15), but Jesus came to give us life abundantly (John 10:10). We can experience that abundant life as we love Him and live for Him, following His ways, including with our relationships (John 15:1–17).

While it’s difficult for us who know incest is a sin and who see it as unacceptable to think about it as having ever been acceptable, we can understand that the population would not have grown if incest did not occur. Within just a couple of generations, there would have been plenty of options for people to not have to marry their siblings or close relatives. We can trust God’s good plan both in populating the earth and in limiting the effects of sin.