A tweet from Ken Ham:

I was on a radio talk program once when a caller asked, “Where did Cain get his wife?” I answered saying Cain’s wife was his sister. The next caller identified himself as an atheist and said, “If you believe that it’s incest and it’s immoral.” I’ve heard many atheists make that accusation over the years.

First, an atheist can’t make a moral judgment like that. If it was truly incest as we understand the word today, then why is incest immoral from an atheist’s perspective anyway? If there’s no God, no absolute authority, then “right” and “wrong” are subjective and can’t be treated as moral absolutes. Also, from an atheist evolutionist’s perspective, humans are just evolved animals, so they could have sexual relationships with anyone (or any animal) as there’s no such thing as marriage (or you define it any way you want), and morality is relative. And yet atheists try to claim Christians have a moral problem with Cain’s wife being his sister! They can’t see their own inconsistencies.

Now the word incest wasn’t even invented until the thirteenth century. It’s a modern word. But doesn’t the Bible forbid marriage between close relations? It does, but the laws against marrying family members were initially given as part of the Mosaic covenant, approximately 2,500 years after God created Adam and Eve. Due in part to genetic mistakes, these laws were necessary to help protect offspring from mutations shared by both parents.

In today’s world, close relations (e.g., brother and sister) marrying would be incest. But originally, there would have been no problem with it. Looking back through history, the closer we get to Adam and Eve, the fewer genetic mistakes people would have, so it would have been safer for close relatives to marry and have children.

Remember, we’re all one race, one family, all descendants of Adam and Eve. So we are all related. When you marry (one man for one woman), you marry your relation. We just don’t marry a close relation today.