Many theistic arguments in defense of the existence of a god involve arguing that certain events that have not yet been fully explained by naturalistic science are unexplainable in principle by naturalistic science. Thus, the only "explanation" available is the supernatural, which according to religious theists establishes a justification for believing in their god. This is a serious misunderstanding of science because something unexplained is not necessarily unexplainable.

In The Atheist Debater's Handbook, B.C. Johnson writes:

Since primitive times, great scientific progress has been made. We have developed explanations, based upon natural causes, for many events previously left unexplained. How, then, can we know that a presently unexplained phenomenon will not turn out to have some natural explanation?

The total class of unexplained events is not necessarily the same as that group which is unexplainable within the arena of natural causes. If it were the same, no scientific progress would ever have been made. This is so because scientific progress by definition consists of developing natural explanations for phenomena previously unexplained in these terms.

To claim that there are phenomena which must be considered unexplainable is to predict what the future course of scientific investigation will or will not reveal. And this would be mere guesswork.

It's always possible, in theory, that naturalistic science won't be able to explain something — but the chances of this being the case don't appear to be very significant.

We don't have any examples of science concluding that something is, in principle, unexplainable; everything which science has investigated thus far has in fact become more and more explainable, more and more understood.

It's only possible that science might fail because the idea of such failure isn't logically contradictory; the possibility of naturalistic science being unable to explain nature is not realistic though. Nevertheless, many theists persist in arguing that some things are unexplainable — but as Johnson points out, this means that they are predicting the future in a way that is completely unreasonable.

Such theists certainly can't claim to know enough about science and the universe to know that such events are unexplainable — that would involve them knowing far more than the scientists studying the phenomena. Scientists acquire knowledge through careful study and testing of the universe we live in; these theists are making pronouncements without the benefit of eitiher.

Johnson labels this "guesswork," but I'd say that it doesn't even amount to that. Instead, it's just wishful thinking. Some theists need for certain phenomena to be "unexplainable" because they can't imagine any room for the existence of their god otherwise. They have an ideological commitment which forces them to argue for something which has no basis in reality.

I'm sure that some clever theologians will be able to come up with justifications for theism even in the context of a fully-explainable natural universe, but right now most theists don't appear able to deal with this and, therefore, continue to rely on gaps in scientific knowledge where they try to hide their god. Those gaps keep growing smaller as scientific knowledge expands, though, and I wonder how much longer they will be able to lean on this fiction.