Every human presupposes the possibility of coming to rationally justified conclusions. Presuppositionalists recognize this fact.
However, any belief ultimately caused without reason is ultimately held to irrationally. Under most worldviews, all human beliefs are ultimately caused without reason. This demonstrates the irrationality of atheism, polytheism, deism, and unitarianism, as well as empiricism.
Note that cause, or answering the "how" question, and reason, or answering the "why" question, are not the same thing. For example, why do you believe that 2 + 2 = 4? A response like "because of electrical processes in my brain" utterly fails to rationally answer the "why" question. It will never convince anyone with any sense that the answer is correct.
Under Christianity, unlike the worldviews mentioned above, human beliefs are ultimately caused by a rational entity, namely, the Christian God. He gives us our first principles. This means that under Christianity it is hypothetically possible for humans to hold to rationally justified beliefs. Our beliefs can ultimately have reason behind them. Under Christianity, the why question can be rationally answered.
Another way of saying this is that if there is no reason for anything, then there is no reason for anything -- including our own beliefs. Or, alternatively, we could say that if everything is just a random accident, then our own beliefs are just random accidents.
Are your beliefs without reason, or is there a God like the Christian God?