An inquisitive student asked her Bible teacher, "How did Cain get his wife when Abel and he were
the only two children of the first human beings, Adam and Eve?" It was a very difficult question for
the teacher to answer. The teacher walked around the room and thought for so long while his
students awaited in silence to hear the most unanswered biblical truth. The teacher cleared his throat, 'huh,' and said in an assertive voice. "That is a very excellent question, and I have also an excellent answer to give you." He paused for a moment, and confidently told the eager children, "I really don't know." "OOhh," grumbled the children. One shouted at the back, "you are a bible teacher, you should know the answer." The teacher answered, "I wish I had known." And he continued. "My dear children, the first question we should ask God when we die and get to heaven is, please God tell me, I had difficulty trying to find out how Cain got his by reading the Bible. Since I am here with you now, tell me how or where did Cain his wife?" The class end without children getting their answer as they desired.
The teacher on his part did an excellent work by telling the children truth that he did not know the answer. He lets God to be one who has all the answers to our difficult questions of life.
If you were a teacher of that class what would be your explanation or answer to the question, where did Cain get his wife? It is not easy to give a satisfactory or precise answer with detail biblical citation to prove your answers as biblical simply because the Bible does not have the answer. And this fact alone is an answer itself. We have to leave it to God and when will God tells us is when we meet him face to face. There are, of course, some ways, in fact two sources of information, in which we can seek information from or give any explanation. They are external and internal sources. Internal source is the Bible and was sought already by the teacher and gave an excellent answer to his students.
The Bible itself must attest for itself. It must reveal, give definite answer where we can pinpoint to texts where we can confirm our answers. In this case, the story of Cain and Abel appeared in Genesis 4. We do not read of any mention of how and where Cain got his wife in verses1-16, but all of a sudden in verse 17 we read of Cain's descendants. We are stunned, how could that be? So there is exactly no internal evidence to really accurately answer how or where Cain got his wife. And we react naturally, where did Cain get his wife? But the fact remains that he has descendants, the question still remains- where did he get his wife? Because of this, there is no concrete textual evidence in the bible- that is there are no internal evidences in the bible. The evidence from internal source will only be deemed authority and proven. Therefore, there is no authoritative answer to the question. The other source- that is the external source we seek to find an answer and an explanation will be only by assumption because it is not an authority.
The external source can be from knowledge derived from researches in archaeology, sociology, mythology and social sciences. Our possible explanation how or where Cain got his wife would be by the following assumptions.
First, it assumed that the biblical writers did not foresee that people in the 21st century will ask such a question. It was not their aim to give details of how and where Cain got his wife; their aim was to tell a story of sin multiplied from a single act of disobedience in the garden. The writers did not think this was the important question for their story, but how sin multiplied was an important question to be answered in the story by highlighting Cain killing his brother and the punishment that came with it.
Second, it assumed that Cain must have married one of his own sisters. This can be proven by anthropological studies that Israelites were endogamy. They practised marriage within their blood clans. For instance, Abraham married Sarah, his half sister. His son Isaac married Rebecca granddaughter of Abraham's brother Nahor (Genesis 24). Jacob married his two cousins, Leah and Rachael. Where else would Cain get a wife if they were the first people? The story is not told in chronological order where events happened in timely order. Cain may have waited till Adam and Eve had other children (Genesis 5:4), he may have married one of his own sisters.
To modern readers and especially inquisitive children who have this sort of question in their mind may see these assumptions illogical and to some extend nonsensical. The biblical writers were not ignorant, but wrote a story that served their purpose as any modern story teller would do. The writer writes to serve his intend and ignores other details he considers unnecessary for his purpose.
On the second assumption, one would really think it is insane or immoral for one to marry his own blood sister or brother. On the contrary, for Israelites, this was the practise- to be pure blooded Israel and not to intermarry with pagan people. It was a practice in the ancient world to keep blood or their species alive.
For human species to be kept alive; Adam and Eve would have no inclination whatsoever to give their daughter to her brother to marry. This practice was common as we read of Lot and his two daughters in the mountain caves of Zoar(Genesis 19:20ff). When the two daughters of Lot saw their father and only male in their confinement aged, they made him drunk and took turns to have intercourse with him to have children just to keep their blood alive. Their act now we may consider it as immoral, indecent and term it incest was lesser evil of the two evils. It would be very evil deed indeed to just live to an old age and die without leaving behind a descendent to keep human species alive. Or for the two daughters to do nothing at all and die and bring human species to end, but it is better to commit what is seen as evil for the good of the continuity of human species. The Ammonites and Moabites were descendents of these acts of the two Lot sisters.
Cain may have been in similar scenario. At another level we just have to see that God' command of multiplication and subduing of earth lies in the core of this human activity. These acts are committed either consciously or unconsciously it is God who allows it to happen so that his desire for the earth to be subdued is achieved as well as his promise of an offspring of the woman to bruise the head of the snake is fulfilled in Jesus Christ(Genesis 3:15). What would happen if this assumption that Cain did not marry his own sister is not probable, where else and how would Cain get his wife? How would it be possible (as the Gospel writers did) to trace Jesus' genealogy to Cain? Or even still how can we be here today asking this very question?