In a purely naturalistic framework, to talk of rationality would be meaningless. On naturalism our cognitive faculties would be geared for survival, not rationality or truth. Therefore, we would have no reason to think any of our beliefs are true, even the belief in naturalism itself. All of what we perceive to be true would be nothing more than electrical impulses in our brains that would only be guiding us for survival. There would be no basis on which to think anything we perceive is true. This is epistemically incoherent and creates a state that is self-defeating for the naturalist.
However, the theist has good grounds to think that they can ascertain truth reliably. If there is a God who has fashioned our minds to be able to grasp truths, then we can be confident in our ability to do so.
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