Have to take issue with this. There is a massive hole in the logic here. Virgin birth and resurrection are irrational (lack of evidence and every piece of biology you care to study says it just doesn’t happen).
Thanks for bringing this up, themadhair, because it provides for useful clarifications. So let's examine this so-called "hole in the logic." You are claiming that Christian belief in the virgin birth and the resurrection is irrational. Why? Because there isn't much evidence for these beliefs and what we know from biology suggests, nay screams out, that such things don't (can't?) happen.
As a Christian, I might ask, "Why think there isn't much evidence?" Christianity, after all, says that there is a way to know the truth of the virgin birth and the resurrection. That way is through what Acquinas calls "the internal instigation of the Holy Spirit." I'll call this the IIHS going forward for brevity. Very briefly (which means this account is extremely truncated and in need of filling out), humankind has fallen into sin. As a result, we are subject to a kind of affective madness such that we love and hate the wrong things. The cognitive upshot of all this is that, although we have a faculty by which we can know truths about God (a divine sense, if you will), that faculty is hindered in its function by our affective madness. So even when we form true beliefs about God in the right circumstances, we are apt to dismiss, reject, or ignore them. At spiritual conversion, God heals (or begins to heal) this affective madness, thereby clearing the way for the proper function of our divine sense. However, that's not enough for us to believe such things as the virgin birth and the resurrection. For even without hindering our divine sense, these things are underdetermined by the available evidence. Yet, believing them is important to salvation and to our properly relating to God through Jesus. So God has provided another means by which we can form true beliefs in what Jonathan Edwards calls "the great things of the gospel." That way is through the IIHS. That is, God Himself testifies to the truth of what is revealed in scripture (or through the preaching of the gospel).
So IF CHRISTIANITY IS TRUE, Christians have a reliable (and rational) means of knowing the truth of doctrines such as the virgin birth and the resurrection: the IIHS (together with the proclamation of the gospel or the reading of scripture). However, IF CHRISTIANITY IS FALSE, Christians don't have this reliable means of knowing the truth of these doctrines. Therefore, holding to them with the firmness most Christians do would probably be irrational. THEREFORE, you cannot claim that Christians are irrational in holding the beliefs they do unless you can show that Christianity is actually false. There are ways of attempting this, but I leave that to you.
Now this must leave you in a quandary. If you follow your line of reasoning above to say that I can only hold this objection due to presupposing the falsity of virgin birth and resurrection then you are begging for the obvious retort – this line of reasoning applies to ANY proposition regardless of its truth value. You don’t believe I am god, well that must be because you have presupposed the falsity of my godliness.
There's a confusion buried in this claim, which I hope my previous comment clears up. The issue is that Christianity, if true, has a rational means of knowing the truth of its doctrines. If you wish to impugn the rationality of Christianity, you must go further than say "there isn't enough evidence." The Christian might well admit that, minus IIHS, historians have little basis for confidence in the resurrection (say). But that need not cause the least spiritual or intellectual crisis for the Christian. For the Christian, again if Christianity is true, has resources that the historian either doesn't have or doesn't avail himself of.
And this is irrational. Faith, in and of itself, does not affect the truth value of any proposition, and believing it does as you imply here is irrational.
Again, I hope my explanations have cleared up the confusion. Just one more point of clarification. It's no part of the Christian claim to say that faith AFFECTS the truth value of a proposition. Rather, Christians claim that faith (what I am calling the IIHS) is a MEANS OF KNOWING the truth value of certain propositions (such as that Jesus was born of a literal virgin and literally rose from the dead).
Altering a belief system so as to render it immune to argumentation while not going the whole hog to align it with reality seems a bit of a cop out to me.
Who has done so? Certainly not I. (And let's not confuse a Christian's explaining parts of his belief system that you may not have heard before or do not accept for "changing" a belief system.)
Ignoring the problem with this as I have mentioned above, I actually believe that starting from an assumption of falsity is the rational approach to take. If I claimed I was god I’d consider you irrational if you didn’t assume the falsity of that proposition to begin with.
Well, that depends on what you're up to. If you and your friends want to mock or pity Christians in the privacy of your own homes, then of course you're free to assume whatever you want about the truth or falsity of Christian beliefs. But if you're trying to demonstrate to the Christian (presumably because you care about her) that there's something untoward in believing as she does, you need to provide some sort of argument that DOESN'T presuppose the falsity of Christian belief. Otherwise, your argument need not have any weight whatsoever with the Christian. It won't provide her any sort of compelling reason to even consider rethinking her position.
In other words, you need to provide her with a defeater for her Christian belief. A defeater is a proposition P which, if understood and believed, puts a person in a position of having to change or abandon her set of beliefs B. For instance, I am in a widget factory and see a row of red widgets passing before me along a conveyor belt. I form the belief "Now there's a fine-looking red widget." The foreman comes along and tells me that the widgets are irradiated by red light. This information casts doubt on my belief that the widgets I see are red. This defeater doesn't show that my belief is false, it simply removes any warrant the belief has. As a result it is no longer appropriate to hold the belief. Call this an undercutting defeater. On the other hand, the foreman might tell me that the widget is actually yellow, but under the special lights they look red. Now I have a rebutting defeater, a defeater that implies that my original belief is false.
So with respect to Christianity, you have two general strategies. You can provide the Christian an undercutting defeater or a rebutting defeater. For an undercutting defeater, you might argue, for example, that contemporary biblical scholarship has reliably shown that the original Christian movement was a hoax. (We have discovered letters from the original apostles detailing in all seriousness how the hoax is to be perpetrated and sustained...) Given the central role of scripture in the Christian's epistemology, that would be devastating. For a rebutting defeater, you might show how Christian (or bare theistic) belief is logically incoherent. The classic problem of evil is a case in point. There may be other strategies, but again, I leave those for homework.
To sum up, I think my point stands. Namely, there are no objections to the rationality of full-blooded Christian belief that do not presuppose the falsity of Christian belief. This rules out the posture of "Well, I can't prove Christianity is false, but even if Christianity is true (contrary to fact, as the objector sees it) it is irrational to believe it."