His answer: It says very little.
He rightly sees, as many in our current culture do not, that explaining the appearance of an idea does not tell us whether or not that idea is true. This is the classic genetic fallacy, after all. It may or may not be true that “you only believe in x because of your personal motivations”, but that tells us nothing about whether or not “x” is true. Mackie sees this, and dismisses the idea that natural histories of religion are, in themselves, reasons to reject the truth of theism.
He goes on, however, to argue that these can be used as a counter to the idea that the existence of religion cannot be explained apart from supernatural intervention.
Again, I agree with him, though I’m left wondering who it is that Mackie thinks has given this argument. He does not tell us, and even goes so far as to complain that theists often point out a fact that runs counter to it: that humans have a natural psychological desire for God.
Of course we do. And this should have signaled Mackie that theists, with very few exceptions, have never argued that the natural desire for God is itself in need of a supernatural explanation.
Mackie seems to think otherwise, and Daniel Denett dutifully informs us that the natural desire for God is a desire for God that is natural–as if that were a revolutionary concept.
In this, and many other places, it seems that those who argue against the truth of theism tend to have a very weak understanding of what theists are actually claiming.