If our brains are the result of evolution then our brains are tuned to believe things that are adaptively useful. The Christian in this book points out that if the truth is that a plant is poisonous then all our brain has to do is believe a claim that stops us from eating it: that plant is poisonous, that plant is sacred, that plant is your uncle reincarnated, eating that plant will give you bad luck. I’m sure the imaginative reader could come up with more. The point is there are more untrue statements our brain might evolve to accept than there are true ones that are still beneficial. With that is mind, we cannot assume our brains evolved as truth-tracking machines.
The atheist points out this is not necessarily true. Taking the plant example, our brains did not guess which plants we should not eat and thus start generating random claims to deter us. We ate the plant as well as many other plants and, by trial and error, we earned the knowledge that we should not eat the plant. And the truth was if you want to be healthy and alive you should not eat the plant. The truth was not, necessarily, that plant is poisonous. I would not be at all surprised if people used to reason that certain plants were magic or protected by God (and even used the Tree of Knowledge of Good and Evil as evidence that some plants are under God’s protection). At a later date that precursors to science progressed our thinking do not eat it to do not eat it, because…
It is actually true that our brains are unreliable. I often share the YouTube video Kalam Cosmological Fallacy to show how often our intuitions fail us. We have methods that shows us our intuitions fail us. Those methods are validated in having a much more reliable and powerful truth-tracking capacity that our brains. The methods are science, logic, maths and language. But our intuitions are not wholly unreliable.
Consider that every truth we might need for our survival actually has 5 possible claims with the same adaptive value. That means there is one true statement and 4 that are untrue but still make us behave in a way beneficial to our survival. Something like the example in the first paragraph of this post. If the brain arbitrarily believed any statement with a survival value then we could expect 20% of the population to believe each of the 5 statements. After all, apparently there is no mechanism to determine true from useful. But we largely agree to one claim: certain plants are poisonous. No part of the population believes the other claims. Something has amalgamated belief in one claim and not the others. That force is truth.