Arlis asked: Who was Cain’s wife Who was on the earth for him to marry?
To put it simply, Cain would have married either his sister or his niece. God created Adam and Eve, who then began having children. God did not create additional human beings.
“And He made from one man every nation of mankind to live on all the face of the earth, having determined their appointed times and the boundaries of their habitation, that they would seek God, if perhaps they might grope for Him and find Him, though He is not far from each one of us” Acts 17:26.
Later on, as the population began to increase, marrying a close relative became a sin called incest. But apparently, this was part of God’s early design as humans were reproducing and beginning to fill the earth. Early on, God allowed the marriage of close relatives. It later became an abomination and continues to be so to this day.
“It came about while Israel (Jacob) was dwelling in that land, that Reuben went and lay with Bilhah his father’s concubine, and Israel heard of it” (Gen 35:22). and “Uncontrolled as water, you shall not have preeminence, because you went up to your father’s bed; then you defiled it—he went up to my couch” (Gen 49:4).
“It is actually reported that there is immorality among you, and immorality of such a kind as does not exist even among the Gentiles, that someone has his father’s wife… I have decided to deliver such a one to Satan for the destruction of his flesh, so that his spirit may be saved in the day of the Lord Jesus” (1 Cor. 5:1, 5).
So, while the first family would have intermarried, it is now prohibited.
Thanks for your question,