This is a fantastic question.

I begin my answer by noting that rationality is overvalued and its capacity overestimated. The ability of the human mind to apprehend what it considers is vast, but not unlimited. Not everything, therefore, can be understood in ways that make apparent sense and align with all the other things that we know. In particular, God would not be God if He could be fully comprehended. For this reason, I side with those who call faith unreasonable.

As a student of Kierkegaard, however, I also note that the central paradox of Christianity, of God present with us, is no more of a paradox than the paradox of existence itself. Why should there be something rather than nothing? Why does our existence mainifest in the shape that it has, rather than in some other form? Why does each of us individually and idiosyncratically exist, and why are we bound by space and time? These are questions that have no rational answers, yet we live with the paradoxes they imply because we lack the ability to do otherwise.

This leads me to what I take to be the key Kierkegaardian insight: The mystery of Christ is not only on a par with the mystery of existence, it is in fact the same mystery. The mystery of why God would enter the universe and suffer and die is the very same mystery as why that universe would exist at all, and why there would be suffering and death within it in the first place.

All this having been said, however, I think there’s a danger in dismissing faith as merely or dogmatically irrational. The believer, I would claim, is not simply a believer in defiance of all evidence –which would indeed make him the thoughtless fanatic of your query.  Speaking as a believer, I would say that God has demonstrated His existence to me with evidence that is plentiful and personally compelling –yet not of a sort that lends itself to conclusive depersonalized proofs.

My aim in making such a claim is not to present a case for God’s existence capable of convincing the non-believer, but to advance the argument that the intrinsic irrationality (or what we might call the “transrationality”) of faith does not necessarily imply that the person who embraces faith must do so in an irrational manner. One may safely assume that the person who believes does so for personally valid reasons, even if those reasons are not easily understood by the non-believer.

This, it seems to me, is the best way to approach the ontological proofs of theological rationalists like Aquinas and Descartes, the apologetics of someone like C.S.Lewis, or the calculated wagers of Pascal and his ilk –not as attempts to equate faith with reason, but rather as ways of demonstrating that faith and reason are at least compatible with one another; and therefore that the embrace of one does not necessitate the destruction of the other.