“If human nature has a predisposition to religious belief (see Day 313), why can’t we explain this as an evolutionary by-product?”
We can’t dismiss our religious disposition so easily.
Religion requires a robust set of cognitive faculties, which is why humans have religion and less intelligent species do not. According to the theory of evolution, the needed faculties would have developed over a long period of time, with different aspects appearing among the different species that led up to the human race.
This is no different than other faculties humans possess, such as those allowing us to use language, do mathematics, or perform scientific reasoning. The cognitive faculties needed for these also would have developed over a long period of time among our ancestors, and we even see traces of these faculties in other species that are alive today.
However, at some point in the history of life, the right set of cognitive faculties appeared in a species—our species, homo sapiens—that allowed us to have the robust and complex institutions of religion, language, math, and science that we display today.
A purely evolutionary perspective would say that each of these is just a product of evolution. The needed cognitive faculties developed (evolved) over time, and so today we have the institutions they make possible.
However, that doesn’t mean that they can be dismissed as “just an evolutionary by-product.” The question is not whether evolution played a role in our being able to have these institutions. That may be granted. The question is what these institutions tell us about the world.
As we cover elsewhere (see Day 353), basic human impulses may be taken as evidence about the world. If our cognitive faculties for mathematical and scientific reasoning developed in a way that allows us to know things about the world, then we must be prepared to acknowledge that the cognitive faculties for religious reasoning are the same.
Put differently, if our mind has the ability to deduce the hypothesis of a triangle or the existence of electrons, it may also allow us to deduce the existence of God.
This is all the more true if, as the Judeo-Christian view holds, the creation of man was superintended by divine providence and there is a God who wants us to know him.