Question: Where did Cain get his wife?
Answer: Let’s see what the Bible has to say. Cain was the firstborn of Adam and Eve. (Genesis 4:1). His brothers, Abel (Genesis 4:2) and Seth (Genesis 4:25) were only two in the first generation born on Earth. Even though only these three are named, Adam and Eve had other children. We find this in Genesis 5:4,
"And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters:"
The Bible does not say when Adam and Eve’s children were born; but, some were probably born in the years before Seth was born, or there would have been no one for Cain to marry. Neither does the Bible say how many children Adam and Eve bore during their long lifetimes (Adam lived a total of 930 years, Genesis 5:5). But, since God commanded them to "...Be fruitful and multiply"...(Genesis 1:28), there were probably many.
Therefore, during the first generation born to Adam and Eve, some men had to marry sisters. As time went on and the grandchildren of Adam and Eve matured, there would have been marriages between brothers and sisters, nieces and nephews, and cousins. This continued to the great-grandchildren and so on.
Many have tried to figure this mathematically. We’ll just call it a population explosion! Some call this incest and say it was against God’s law. This is not true, as it was not until approximately 2500 years later, in the time of Moses, God forbade brother and sister (and close relatives) to marry. Read Leviticus, Chapters 18 through 20.
We should also point out another fallacy. Cain would not have needed a mark if he could flee to another race of people who did not know him and could have cared less; since they would not be relatives. He was afraid (Genesis 4:14,15) because he feared his "relatives" would take vengeance against him for his brother, Abel’s, death. There was only one "race" of people on the Earth and they were ALL descended from Adam and Eve!
One final point! Cain did not flee to Nod because there were many people there who could help him build a great city. He went there to build the city with the help of his descendants listed in Genesis 4:16-24. Where did Cain get his wife. He married his sister, or niece; and took her with him to the land of Nod and there they had children!
According to the Bible, we are all descendants of Adam and Eve and the only ones who can benefit from the salvation provided by God (Romans 5:12). Only by accepting the perfect sacrifice of another descendant of Adam (Luke 3:23, 38), the Lord Jesus Christ, can we be saved. 1 Corinthians 15:22 states:
"For as in Adam all die, even so in Christ shall all be made alive."
To be "in Christ" means you have accepted Him as your personal Savior. I hope you have done this.