If reason is reduced to mere physical processes occurring inside the brain, then everyone reasons, whether it's Christians who celebrate Jesus during Christmas, or "atheists" who complain against Christians for celebrating Jesus. Both groups have physical processes going on inside the brain, so how does one determine which one is being reasonable or not? In other words, in a physicalist paradigm, whose brain process becomes the determining factor of reason, say, between a Christian celebrating Jesus or an "atheist" crying about it?
Also, you fail to understand the metaphysical nature of concepts, such as reason. Reason has no physical element to it. It does not take up space, nor can it be observed empirically (by a microscope or something else). For a physical object like a box, we can examine its tangible qualities, such as its weight, length, texture, etc. With other things like reason, we cannot do any such thing. Reason is an immaterial entity. It makes no sense to say "Reason weighs 13 lbs.," or "I took Reason out of my pantry today."
But "atheists" have no way to account for immaterial realities like reason in terms of their worldview because they only allow empirical/physical entities to be observed as true. They have to borrow assumptions and terms from the Christian worldview in order to even appeal to reason. And in so doing, they attest that God is true, even if they suppress knowledge of His existence.
Did you even pay attention to the context of the discussion?
