Leviticus 18:9 says, "You shall not have intercourse with or uncover the nakedness of your sister..." With this being the case, how did Adam and Eve's children propagate the race without having intercourse with their siblings?"
Good question and one that deserves an answer! Please keep in mind, however, the Scriptures do not directly address a lot of issues in terms of "chapter and verse." We have to pull from many sources to answer some questions; and some inquiries, to be totally honest, will not be answered on this side of eternity!
Here goes! Some have tried to "excuse" God by teaching that He probably created other human beings right after He created Adam and Eve. We read in 1 Corinthians 15:45 that Adam was "the first man." God did not start by making a whole group of people--men and women. Defenders of the gospel must believe 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. Romans 5:12 tells us that "even through one man sin entered into the world, and death by sin, and so death passed on all men inasmuch as all sinned."
The Bible makes it clear that only the descendants of Adam can be saved. Romans 5 teaches that we sin because Adam sinned. Since Adam was the head of the human race when he "fell," we who were in the loins of Adam "fell" also. If God created other human beings in addition to Adam and Eve, then the gospel message is rendered false. So please don't buy into this convenient, less substantiated argument.
Let's face it. If we work totally from Scripture, without attaching our personal prejudices or other extra-biblical ideas, then we have to accept the fact that back at the very beginning, where there was only one generation, brothers would have had to marry sisters or there would be no more generations! The Bible does not tell us how many children were born to Adam and Eve. However, considering their long lifespans (Adam lived 930 years), it seems reasonable that there were many. Famed Jewish historian Josephus wrote that "The number of Adam's children, as says the old tradition, was 33 sons and 23 daughters."
Remember, too, that sibling ages could be separated by hundreds of years! To fully understand this concept, we have to quit thinking like traditional "Ozzie and Harriett" Americans. We are not told when Cain married or any of the details of other marriages and children, but we can say with some degree of certainty that some brothers had to marry their sisters at the beginning of human history. News flash I realize I'm challenging religious mindsets; however, God continued to do "strange things" throughout redemptive history. Do the names Hosea (prophet) and Gomer (prostitute) ring a bell?
Most people immediately reject the conclusion that Adam and Eve's sons and daughters married each other by appealing to the verse you quoted in Leviticus 18. Some say you cannot marry your relation. But if this first generation of humankind did not marry their relations, they could not marry a human! Understand something. The law forbidding marriage between close relatives was not given until the time of Moses. Remember that Abraham married his half-sister (Genesis 20:12). God blessed this union to produce the Hebrew people through Isaac and Jacob. It was not until some 400 years later that God gave Moses laws that forbade such marriages.
Today, brothers and sisters are not permitted by law to marry because their children have a reasonably high risk of being deformed. Besides, that would be gross anyway! Just as the human lifespan shortened as sin continued to ravage mankind, so the human gene pool surely degenerated. Adam and Eve possessed no genetic mistakes. They were created physically perfect. Everything God made was "very good" (Genesis 1:31). Their genes were perfect--no mistakes. But when sin entered ht world, God's perfect creation began to generationally degenerate. Cain was in the first generation of children ever born. He, as well as his brothers and sisters, would have received virtually no imperfect genes from Adam and Eve, so their offspring would have been genetically sound. Sadly, sin began to work its destructive force against the beauty of God's creation, and by the time of Moses, degenerative mistakes would have built up in the human race to such an extent that it became necessary for God to forbid brother-sister marriages. Also, there were plenty of people on the earth by then, and there would have been no reason for close relations to marry.
I believe this argument is plausible. Regardless, the Bible is still God's Word and God's ways are perfect. I still say we underestimate the toll that sin has taken on humankind and this planet. This venue does not allow me to go into more detail, but I'm thankful for the information provided by www.bibleexplorer.com. I drew heavily from their research. If your curiosity has peaked, log on to that site and type in "Where did Cain get his wife?" Questions like yours make me appreciate the integrity of the Scriptures even more!
Perhaps you have a question that you'd like for me to address? Send it to me by facebook or email it to: loomisrl@windstream.net.
