If God does not exist, then nothing has purpose. If nothing has purpose, nothing has to be a certain way.
Consider the intellect. Everything we know comes through our senses, and is processed by our mind. All of our perception is based on data transmitted by the senses and that data is interpreted by the brain.
Now, consider this “brain” and the “senses”. Let’s presume, for the sake of the argument, that these two faculties are wholly material things. They are machines made of parts (nerve cells). In fact, they are made of billions of billions of parts, as huge numbers of neurons make up the brain and senses.
Let’s consider what we have. We have an enourmous machine composed of billions of parts. If any of those parts are missing or non-existent in the first place, the result can be unpredictable (a simple facet of mechanics in general- missing or damaged parts can affect the result in many different ways).
So, we have an enourmous machine composed of billions of living cells. Now, we presume that those cells interpret the incoming data into an accurate image of the subject. We can’t know this for sure, because we can’t step outside of our senses to check.
So in the end, we are left with the proposition that, due to undirected, purposeless forces, the body of man assembled billions of parts into a machine that can know all reality and comprehend the question of whether a transcend being exists or not. This is just amazingly fortunate. We don’t know for sure whether those billions of parts assembled in that way. We’ll just assume we have the necessary parts to answer the God question and live our life according to that conviction.
This is the irrationality of atheism.
If atheism is true, there is no reason to expect our mind to have developed in the way where we could know that truth, and there is no way to check whether it has developed in that manner. If atheism is true, then we almost certainly would not be able to know it. There is no reason our minds B]must accurately think about a non-existent entity, so why would we assume that our minds for some odd, undirected reason aquired that capability?
Of course, if you believe that God arranged the billions of parts in a way where we can know reality, this problem is overcome.
Below is a quote from Darwin which expresses a particular take on this issue: why would we trust our convictions any more than we trust the convictions of a monkey?
With me, the horrid doubt always arises whether the convictions of man’s mind, which has been developed from the mind of the lower animals, are of any value or at all trustworthy. Would anyone trust in the convictions of a monkey’s mind…?
Letter to William Graham, Down, 3 July 1881. In The Life and Letters of Charles Darwin Including an Autobiographical Chapter, ed. Francis Darwin (London: John Murray, Albermarle Street, 1887), Volume 1, pp. 315-316.