Today's question is one that has become a commonly used challenge against Christianity. You'll often find this question on Christian apologetics sites, and those who enjoy attacking Christianity will often raise this question. But I'm not going to simply regurgitate some answer that you could probably find yourself by picking off the first Google search result. I've been asked this question a few times myself, and (as always) I'm going to share my very own thoughts with you.
What makes this question seemingly problematic for Christianity in the first place? Well, the Bible tells us that Adam and Eve were created, and that they were the only people on the Earth. If they had a son, (Cain for example), who was there for him to marry since marriage is supposed to be with someone outside of the family? The Bible clearly prohibits incest between a brother and sister (Leviticus 20:17). So you see we have a “catch 22” - how can the human race continue without committing this sin? It is, in fact, a command of God to “be fruitful and multiply” (Genesis 1:28), and yet it would seem that obedience to this command will mean disobedience to another. How can God create such a moral dilemma? His own commandments seem to be in conflict.
I want to comment first of all on one or two arguments that have been put forward to explain this problem. There is the idea, first of all, that since the word “Adam” literally means “man”, that Adam was not one individual man, (nor was Eve one individual woman), but that when God created “Adam”, it means that God created a whole number of men and women; “mankind”. And the other idea is that the whole story of Adam and Eve, and in fact, the whole story of Creation, is not to be taken literally in the first place. Rather, it is a sort of poetic story to express some theological ideas, but does not describe actual history. However, I reject both of these ideas. The book of Genesis itself traces the family tree from Adam and Eve. So does the first chapters of Chronicles, and so does the genealogy of Christ in Luke 3. That is to say, according to other references in Scripture, Adam and Eve are considered to be real individuals along with the rest of those named in these genealogies. At what point in the genealogies do we begin speaking of actual people, if the first names are fictional characters, or "multiple characters"?
But what does this leave us with? Surely the only possibility left is to admit that Cain must have married his sister. (Or at least one of Adam and Eve's sons had to have.) Well, though the Bible doesn't tell us explicitly who Cain's wife was, this seems to be the only possibility to me, and this is what I believe did happen. How can this be, that God would create a situation where committing the sin of incest was the only way forward? Well, here I think is a good opportunity to consider the purpose of the Law of God. The Law of God was intended to be examples of righteous living. It is supposed to take God-given wisdom to know how to live righteously, and the Bible is clear on that; all throughout the Wisdom Literature of Proverbs, for example; but we generally see this everywhere in Scripture. So this is to say that living righteously is not as cut and dry as looking up the do's and don'ts of the Law. Jesus spoke to the Pharisees, for example, about the Sabbath, saying that while the Law prohibits work on the Sabbath it also requires the priests to perform their duties in the temple on the Sabbath. (Matthew 12). So again we see an apparent conflict in the Law of God; that no one is supposed to work on the Sabbath, and yet the priests are required to. But if we understand the Law correctly, there are overriding factors. The primary overriding factors are love and mercy. For example, Jesus was rebuked for healing on the Sabbath; but Jesus pointed out that if a man's ox fell into a ditch on the Sabbath, its owner would work to pull it out. (Luke 14). If mercy for an ox overrides the Sabbath law, how much more should mercy for a man who needs healing override the Sabbath law? And so if we apply this to the problem of Cain's wife, we can say that there is an overriding circumstance... there are no other women than sisters.
We have to be careful, of course. The true motivations of our heart are thoroughly and perfectly judged by God. We cannot lightly claim that something is a valid “overriding circumstance”. The Pharisees, for example, would donate gifts to the temple (referred to as “Corban”). This sounds terribly pious, but Jesus rebuked them because they were giving to the temple what should have gone towards the care of their elderly parents. (Mark 7). So “donating to the temple” (or to the church) is not a valid excuse for failing to honour your father and mother. We can begin to justify all kinds of things if we're not careful. Was it right or wrong for the daughters of Lot to get him drunk and rape him; their reason being that they lived isolated from any other men whom they could legally marry? Was it so important that they have children at all? The world is full of moral dilemmas, and it is God's desire that we seek His guidance and wisdom to know what we ought to do. Christ and His Word are to be our guiding light in this world.
For you were called to freedom, brothers. Only do not use your freedom as an opportunity for the flesh, but through love serve one another. (Galatians 5:13)