It is quite the oddity for Christians to claim that, if our brains are the products of an unguided process, we must then forever remain uncertain about our brains’ reliability.
You doubt your brain’s reliability? Simply test your brain. We all have goals. Simply divide your goals in half. Then for one half, use your brain. For the other half, don’t use your brain. Which method best accomplished your goals? To what degree did using your brain accomplish your goals? To the degree that your brain worked, to that degree you are justified in believing it will continue to work. Pretty simple.
Consider finding a compass on the ground in the woods. It appears to accurately point north based on its pointing in the same direction that most of the moss on the trees face. Now consider a friend telling you that you can’t ever know whether the compass is reliable unless you know the company that made the compass. Nonsense, right? You simply test the compass. If it reliably gets you home through the woods night after night, it is reliable. Nothing difficult here. Very basic.