If humans originally multiplied by Adam and Eve’s children having sexual relations with each other, wasn’t that sin?

Q. When Adam and Eve started a family, how did the children multiply the earth without having relations with each other? After the flood, how did Noah’s family multiply the earth without having relations with each other? How could this be allowed and later a sin? I cannot wrap my head around this. Isn’t sin sin?

This is an excellent question. The Bible does indeed teach that the human race is descended from a single couple and that it is sinful to have sexual relations with a close relative such as a sibling.

But I think the reason why we have a problem trying to wrap our minds around this is that we tend to feel that if there are any circumstances at all under which an activity would not be sinful, then it must actually be not sinful in all circumstances. But that is not a necessary conclusion. There are, in fact, some extraordinary circumstances in which activities are justifiable that would not be justified in general, and this does not reduce morality to “situation ethics” in which the right thing to do is simply the right thing to do in a given situation.

For example, in her book The Hiding Place, Corrie ten Boom tells about the moral dilemma that she and her family felt about lying in order to protect the Jews they were sheltering. She describes how some Christians who were sheltering Jews felt they could not, under any circumstances, say that something was true if it was not true. And so when Nazi soldiers came to their homes and demanded to know if there were any Jews there, they would admit that there were and turn the Jews over to the Nazis, who put them in concentration camps and killed many of them. Corrie and her family, for their part, would lie and say that they had no Jews in their home. By doing that, they saved many lives. I think that many Christians who believe and follow the Bible’s teachings about sin would feel that they did the right thing. (For a fuller discussion, see the series that begins with this post: Does God let us use deception for a good cause?)

To take up an even more exceptional example, consider the much-discussed case of the charter flight that crashed in the Andes in 1972. The survivors of the crash recognized once search-and-rescue efforts had been abandoned. After eating all of the available food and even trying to eat cotton and leather from the plane’s seats, they ultimately realized, after agonizing reflection and conversations among themselves, that they could only continue to survive by eating the bodies of the passengers who had died in the crash. They did so, and the world was amazed when, two months after the crash, two of the survivors succeeded in hiking over a glacier and down into a sparsely inhabited valley to get help.

The survivors were all Catholic, and a priest heard their confessions. The priest told them that they would not be damned for cannibalism, given the extreme situation that they had been in. Several of the families of passengers who had died in the crash said they were certain that their loved ones would have wanted to give their bodies in order to save the lives of the others. This episode is still widely discussed. But even those who say that the survivors did the right thing do not argue from it that cannibalism should be permitted under anything other than such very extreme circumstances.

So, to return to your question, let me respond to it with another question, which Christians of good will, with equal commitments to the authority of the Bible, might answer differently. Suppose a disaster struck the earth and the only two people who survived were a brother and sister of child-bearing age, who knew for certain that they were the only humans left. Would they be morally justified in marrying one another and having children in order to continue the human race?