Summary: Some have adopted the viewpoint that Cain's wife belonged to a separate group of people with little or no connection with Adam and Eve. However, the Gospel message requires that all have descended from Adam and Eve..
Why is it important?
Many skeptics have claimed that, for Cain to find a wife, there must have been other ‘races’ of people on the Earth who were not descendants of Adam and Eve. To many people, this question is a stumbling block to accepting the creation account in Genesis and its record of only one man and woman at the beginning of history—a record on which many Old and New Testament doctrines depend.
Defenders of the gospel must be able to show that all human beings are descendants of one man and one woman (Adam and Eve) — as only those people who are descendants of Adam and Eve can be saved. Thus, believers need to be able to account for Cain’s wife and show clearly that she was a descendant of Adam and Eve.
First, let’s establish that Adam was the first human ever created.
Genesis 2:7-8, 18-20, “Then the LORD God formed man of dust from the ground, and breathed into his nostrils the breath of life; and man became a living being. And the LORD God planted a garden toward the east, in Eden; and there He placed the man whom He had formed…Then the LORD God said, ‘It is not good for the man to be alone; I will make him a helper suitable for him.’ And out of the ground the LORD God formed every beast of the field and every bird of the sky, and brought them to the man to see what he would call them; and whatever the man called a living creature, that was its name. And the man gave names to all the cattle, and to the birds of the sky, and to every beast of the field, but for Adam there was not found a helper suitable for him.”
The New Testament affirms that Adam was the first man created -
1 Corinthians 15:45, “So also it is written, ‘The first MAN, Adam, BECAME A LIVING SOUL.’
1 Timothy 2:13, “For it was Adam who was first created, and then Eve.”
Now, it is extremely important to acknowledge that Adam was the first man created. This is because the Gospel is predicated on that point. Romans 5:12 reads - “Therefore, just as through one man sin entered into the world, and death through sin, and so death spread to all men.” The physical death penalty, which Adam received as judgment for his sin of rebellion, also passed on to all his descendants. Since Adam was the head of the human race when he ‘fell,’ we who were in the loins of Adam ‘fell’ also. Because a man brought sin and death into the world, all the descendants of Adam need a sinless Man to pay the penalty for sin and the resulting judgment of death.
God provided the solution—a way to deliver man from his wretched state. Paul explains in that God provided another Adam! The Son of God took on a human nature. Jesus Christ, in His humanity, was a descendant of Adam. He thus became our relation! He is called ‘the last Adam’ (1Corinthians 15:45) because he took the place of the first Adam. He became the new head and, because he was sinless, He was able to pay the penalty for sin: 1 Corinthians 15:21-22, “For since by a man came death, by a man came also the resurrection of the dead. For as in Adam all die, even so in Christ shall all be made alive.” Christ suffered death (the penalty for sin) on the cross, shedding his blood so that those who repent of their sin of rebellion and put their trust in His work on the cross can be reconciled to God. Romans 5:17-19, “For if by the transgression of the one, death reigned through the one, much more those who receive the abundance of grace and of the gift of righteousness will reign in life through the One, Jesus Christ. So then as through one transgression there resulted condemnation to all men, even so through one act of righteousness there resulted justification of life to all men. For as through the one man's disobedience the many were made sinners, even so through the obedience of the One the many will be made righteous.”
If Adam was not the first man and father of the whole human race, the Gospel fails to make any sense. If there are human beings that do not descend from Adam, then they should not be subject to Adam’s penalty of physical death. And, if they are not subject to sin and death, they have no need for a Savior.
Since the Bible describes all human beings as sinners, except the God-Man Jesus, and we are all related (Acts 17:26, “He made from one, every nation of mankind to live on all the face of the earth.”), the gospel makes sense only on the basis that all humans alive and all who have ever lived are descendants of the first man Adam. If this were not so, then the gospel could not be explained or defended.