Notwithstanding any of the foregoing, I think one point remains left unsaid. Some light needs to be shed on MacCullough's own motivations. Indeed, Thirsty helpfully cites E.H. Carr to the effect of needing to understand why a historian selects what he or she does, why they deploy their evidence in the way they do, and what it is that motivates them to write in the first place. You may think, for example, historian X really botched things up because he obviously did not consider document Y (most likely something you have a passing familiarity with) from perspective Z (which you probably like to think of your intellectual standard). But that is not to say historian X is wrong; unless, of course, you perceive yourself the sole arbiter of historical truth. The canons of historical criticism have a stringent flexibility about them, which I take as an implicit recognition the thing under investigation (e.g. the thoughts or actions of someone or some group of people) only comes down to the present in bits and pieces--unlike, say, the canons of inductive reasoning, which are intransigently inflexible.
Carr's admonishment is a helpful starting point, because I don't think Thirsty's sensitivity to MacCullough's self-understanding of the task of a historian is what it could be. Thirsty's take on the matter is fairly clear: historians the world over ought to aim at a sort of factual accuracy that grasps the human contours of the situation. It will not do, in other words, to let your own preconceived ideas about the True, the Good, and the Beautiful loose on the historical stage. Other people will have their own take on those matters as well. The careful historian is sensitive to character of the material, technological, artefactual, and intellectual context of their subject; and they are agnostic in matters of transcendent concern. Or, if Thirsty would grant me a little interpretive license, the historian's transcendence is immanent in a human form, in much the same way the prologue to the Gospel of John presents God in human form; the difference being that the historian's concern is never focused on a single man, but embraces, at least in principle, every human being who ever lived.
MacCullough's motivations are somewhat different, and if considered carefully, may actually illuminate way he either misunderstands or misrepresents Catholicism. His motivations are on display where he discusses his reason for subtitling his history of Christianity: The First Three Thousand Years. The subtitle contains a dual-reference: to the Jewish paternity of the Christian faith, and also to its yet unwritten future.
In fact, the self-identified 'candid friend of Christianity', MacCullough, writes more for the future than he does to understand the past. G.K. Chesterton observed somewhere I can't quite recall, while the past is as wide as humanity, the unwritten future is as narrow as oneself. The version of Christianity's history MacCullough puts forward is tailored to his unabashedly personal vision of the way things ought to be. He says,
My aim has been to seek out what I see as the good in the varied forms of the Christian faith, while pointing clearly to what I think is foolish and dangerous in them.Which is immediately followed by his pronouncement of an anathema on fanaticism of all sorts.
The task MacCullough gives himself as a historian is that of exposing religious insanity and irrationality. He is the quintessential post-Kantian critic of consciousness, who selects from the evidence according to the needs of a future, which he intends to write. This doesn't mean the history he writes is flat out wrong; only that it bends one way and not another.
The bent lends him to interpreting the existence of the Catholic Church as a community ruled from the top-down. The post-Kantian critic of consciousness is trained to expect that dogmatists will impose their dogmas on others, and that what applies in the realm of thought, also goes in the realm of action. Hence the government of the Catholic Church must also be top-down, despite all the evidence to the contrary.* With an eye to the future liberation of humanity, the critic will expose the irrationalities of the dogmatist's claims. If the particular bits of evidence do not conveniently line up with the cosmopolitan intention of the critic, that is perfectly fine. They are writing for the future, where irrationalities have been written out of the story.
Now, Thirsty can complain all he wants about MacCullough's treatment of the evidence. That does not change the fact that he and MacCullough have two very different ideas about what it means to read the evidence.
So instead of enumerating MacCullough's many scholarly sins, none of which I have any reason to take issue with, one actually has to persuade him of the need to confess them at all. MacCullough has never, to my knowledge, feigned obedience to Rome. Like so many others persons in these strangely halcyon days following a pope's voluntary abdication, however, he has found reason to express an interest in the future direction of the Catholic Church.
As that Church professes to embrace the whole human race--potentially, if not yet in actuality--MacCullough's caricatures would seem to be its cross to bear; which throws Thirsty's conclusion, 'Know yourself and know your enemy, and all that', in sharp relief.
Know yourself, and let the rest work itself out.
*Richard Dawkins and Christopher Hitchens made this same error when they assumed the pope was ultimately responsible for the sexual offenses of priests perpetrated against minors and set about to issue a warrant for his arrest from a British court, just in time for a papal visit. The warrant was never issued.