It would be helpful to clarify in what sense we're using the word "rationality" in this discussion. Here are a few characterizations of rationality that have been proposed (by philosophers, at least).
Aristotelian rationality
Aristotle says humans are rational animals. Here the term "rational" seems to point to a property that distinguishes humanity from other animals. This property is the possession of ratio, the power of reason. Humans, unlike at least many other animals, have concepts and can hold beliefs; they can reason, reflect, think about things, even things far removed in space or time; humans are (or can be) knowers. Rational powers can come in degrees. We ordinarily think of ourselves as superior to other terrestrial animals, although we are usually prepared to say that some of them have at least rude powers of reason. There may also be aliens who far surpass us. So is the skeptic denying that a creature rational in this sense can accept Christian belief? Presumably not: given the many millions of rational animals who do accept it, that question has too easy an answer.
Rationality as proper function
So rational animals can and do accept Christian belief. But perhaps it is only malfunctioning rational animals who accept it, those whose rational faculties are dysfunctional or impeded in their function. A person in the grip of mania may form all sorts of weird beliefs; once the mania fades, we might say that rationality is restored. But during the mania, the person's cognitive establishment is dysfunctional. Therefore we can say that rationality is a matter of proper function, the absence of pathology. And irrationality is a matter of malfunction of (some of) the rational faculties, those by virtue of which we are rational animals.
Internal rationality
Here we need a distinction between internal and external rationality. Let's look at internal rationality first. Internal rationality is a matter of proper function of all belief-producing mechanisms "downstream from experience."
Experience
Experience comes in several varieties. First, there is sensuous imagery (sights, sounds, smells, etc.). Such imagery is important in perception; perceptual beliefs are formed in response to sensuous imagery.
There is other experience that goes with belief formation. The formation of memory beliefs is often accompanied by phenomenal experience, even if it is fragmentary, fleeting, indistinct, hard-to-focus. You remember attending Frank's party last year; the imagery you have is faint, indistinct, partial and fleeting. But there's another kind of experience present. The belief that it was Frank's party somehow seems right, acceptable, natural. It forces itself upon you; it seems somehow inevitable (what's the right word?). The belief feels right; it feels different from the belief that you were at Susan's party (to which you weren't invited). The same sort of experience goes with a priori beliefs. The proposition that 2 + 2 = 4 has no sensuous imagery associated with it at all (at least for me). But it still feels true; more, it feels necessarily true. Thinking about this proposition feels different from thinking about 2 + 2 = 7. Something about the latter belief feels rejectable, wrong; more, it feels necessarily wrong.
This second kind of experience is called doxastic experience because it always goes with forming beliefs. Internal rationality includes, in the first place, forming or holding the appropriate beliefs in response to experience, including phenomenal imagery and doxastic experience. With respect to the first, if I am appeared to in the way that goes with seeing a grey elephant, I don't form the belief that I am perceiving a Mozart opera.
Forming the right beliefs in response to doxastic experience is more interesting. A pathological skeptic might have the same sort of doxastic experience as the rest of us but still be unable to form the appropriate beliefs. I might be appeared to in the way that goes with seeing an apple fall from a tree. Out of pathological skeptical caution, I cannot believe that an apple is really falling. After all, it could be a cunningly contrived illusion, or I could be dreaming, or a brain in a vat, or.... This sort of response is precluded by internal rationality. On the other hand, take a person who believes his head is made of glass. That sort of response is not precluded by internal rationality. Perhaps this madman is subjected to overwhelming doxastic experience. The proposition that his head is made of glass is just utterly obvious to him, as obvious as 2 + 2 = 4. Then the problem lies with the seeming, with their having this experience. However, given their doxastic experience, proper function requires (ceteris paribus) forming the belief. This person is externally irrational but internally rational.
Coherence, and responsibility
Lastly, internal rationality requires coherence. The idea is that if a person believes his head is glass, if he is coherent, he will also not believe that his head is flesh and bone. Further, an internally rational person will draw correct inferences from his beliefs. The man who thinks his head is glass will realize that playing football without a helmet is, for him at least, extremely dangerous. Still further, internally rational people choose appropriate courses of action. Given that you do believe your head is made of glass, you will avoid bumps. Finally, internal rationality requires preferring true beliefs to false ones, looking for evidence when that is appropriate, and in general trying to be epistemically responsible.
External rationality
External rationality requires proper function with respect to the formation of sensuous and doxastic experience. An externally rational person forms sensuous and doxastic experience in the right way. The sensuous and doxastic machinery itself is functioning aright.
Christian belief irrational?
Although a few might demur, most would concede that Christian belief can be held by people whose rational faculties are not malfunctioning, at at least not so as to involve clincial psychoses. Of course, Christian believers hold down jobs, some even as academics. But this doesn't settle the issue. Freud says Christian belief is the product of wish fulfillment, presumably not a cognitive capacity designed to acquire true beliefs. Marx says it's the result of a social malaise, the product of dysfunctional society. So maybe there's a non-question-begging way to say Christianity is irrational here, but it awaits something more than the raising of the possibility. The skeptic still owes us an argument.
Rationality as the deliverances of reason
Among what we know are self-evident propositions. Roughly, a proposition is self-evident if it is so utterly obvious that we can't even understand it without seeing that it is true. Examples include propositions like 7 + 5 = 12, and If Tom is taller than Jane, and Jane is taller than Matt, then Tom is taller than Matt. So taken, reason is our capacity to know the truth of self-evident propositions. This faculty also enables us to see that one proposition entails or implies another. The deliverances of reason, therefore, are self-evident propositions together with propositions self-evidently entailed by them. A proposition is rational, then, if it is a deliverance of reason. A proposition is irrational if its denial is among the deliverances of reason. So 7 + 5 = 12 is a deliverance of reason. It is rational. 7 + 5 = 13 is a denial of 7 + 5 = 12, so it is irrational. On this showing, lots of propositions -- contingent ones -- are niether rational nor irrational. Caesar crossed the Rubicon is not self-evidently true, so it is not a deliverance of reason. It is not "rational" in that sense. However, neither its denial, It is not the case that Caesar crossed the Rubicon. Rationality, taken in the sense under discussion here, does not attach to such propositions.
Is Christianity rational in this sense? No. The central truths of Christianity are not self-evident. Nor can they be deduced from what is self-evident. Of course, that's nothing against Christian belief. The same holds for what is taught by historians, physicists, and evolutionary biologists.
Well, is Christianity irrational? Do the truths of Christianity deny or entail the denial of a deliverance of reason? If Christian beliefs were irrational in this sense, that would indeed be something against them. Some skeptics have taken this line. The classic deductive problem of evil is an example. Others have argued that Christian beliefs such as atonement or incarnation are self-contradictory. The results of this type of argument, though, are inconclusive. Everything depends on the precise formulation of the doctrine we are considering. Some of these formulations may indeed be inconsistent; but it is hard to find a really inconsistent formulation that is widely believed. Further, if a Christian comes to realize that he believes in an inconsistent version of a doctrine, he can easily replace that formulation with one that is consistent. However, those who press the idea that Christian belief is irrational are not really saying that Christian belief is merely inconsistent. Generally, the claim is that, even if Christian belief is perfectly consistent, there is still something seriously wrong with it. So clearly it is not this kind of rationality at issue when skeptics charge Christian belief with irrationality.