The reason why it annoys me, and has always (both before and after I joined the Christian religion) was the sheer sense of incoherence involved in the matter. Things in life, things in society, happen for a reason. Christianity developed out of the forms and laws and customs of Hellenized Jews, and so has Greek and Jewish elements in it, but all this was strained through the legal and moral percepts of Roman law and philosophy. The attributes stereotypically associated with "generic fantasy Middle Ages Church settings" are all Christian attributes because  THERE AIN'T NO SUCH THING AS A GENERIC MIDDLE AGES SETTING. The Middle Ages were Christian, not generic.

Specifically, they were the admixture of Northern barbarian cultural values mutating and being mutated by Italian and Greek civilization that they both admired and imitated and overran.

Knights are Christian. Heavy cavalry units from any other era and civilization simply are not knights, not the Paladin of Charlemagne, and simply do not have the other attributes of knighthood.

When Gary Gygax or George RR Martin attempt to construct a Faux Middle Ages setting, with castles and princesses and dragons and so on, but they don't want to offend anyone, so they want to leave the Christianity out of it, they also remove any reason for the institutions being and acting the way they do.

Tolkien came the closest to having a Faux Middle Ages that was actually medieval in feel and flavor, but the way he did it was by putting in place every element in his Middle Earth that had a parallel element in real earth, so the structure still made sense. Instead of the fall of the Roman Empire, we have the Fall of Numenor. Instead of the split of the empire East from West, we have the split of the Kingdom is Isildur, North from South. Instead of Constantinople, was have Minas Tirith. Instead of the paynims, we have the southrons, who look and act just like them. Instead of the Corsairs from north Africa, conquered Christian lands, we have the Corsairs of the Umbar, who once were Numenoreans, but then fell under the sway of the Dark Lord. Instead of the Turks, we have the Orcs.

But Tolkien carefully (almost unrealistically) removes any mention of religion from his world. We have a few references to Elbereth, the Star-kindler of the Elves, or to Eru, The One, and we can glean the names of the archangels from the SILMARILLION. But we do not see any services being held for soldier died on the battle field. There are no monkish or priestly orders, no chapels, no temples, no Sybil of Avernus and no Oracle of Delphi.

When I was reading GAME OF THRONES, the parallel between the crusading religion of the Seven and the "Old Ways" who carved faces into trees is a parallel to the relationship between growing Christianity and dying paganism in the post-Julian-the-Apostate days onward. But why does the religion of the Seven oppose other forms of worship? Why do they have priesthoods? In Rome and Athens, the priests were state functionaries who were usually members of royal or noble families, not a full-time profession involving oaths and special uniforms. There were no archibishops of Zeus and no Pope of Jupiter. Even the religion of the Mohammedans (which is as close to Christianity as you can get, once you remove the pessimism about man's estate, and the humanity of having a human god) did not have these Roman bureaucratic and militaristic hierarchic ideas.

My objection to such things is that they never seem to hang together, and they never seem to copy the real religious practices of other real religions, such as Buddhism or Hinduism or the semireligious philosophy of Confucianism. That is because the Christians evolved something unique in history. Other religions have priests, and some oriental religions even have things like holy hermits who practice asceticism. The Roman genius for authority and organization (which has both its good and bad aspects) developed things like the Office of Pope and the canonical Holy Books. The holy books of other religions simply do not have the legal imprimateur of their established Church hierarchy, because there is no other institution like the Church. The closest parallel is to priestly families, as Levites among the Jews or Magi among the Zoroasterians, or a priestly caste, as Brahmins among the Hindu.

Its bogus. It is as bogus as shows that want to have Ninja, but don't want to take place in Japan, so they invent the Ninja of Arabia! or the Norse Ninja of Iceland! Yes, that ancient Viking assassin-clan that used magic and deception, and the rigorous training methods of Viking Buddhist monks, in the name of Odin!

Bogus as a three dollar bill. When the Gary Gygax of AD 4008 decides to make a 'Generic' pre-atomic war role playing game, the setting will always be the 'generic' federated republic composed of 50 smaller states who broke from the ethnic and monarchic ancient regimes of the Old World: these Genericans will always have Nine Supreme Judges robed in place, and a Senate like the senate of Rome, and a House of Representatives, modeled on the House of Commons in Britain ... BUT ... so as not to offend anyone, there will be no Supreme Executive officer of the Republic, indeed, no executive branch at all. There will never once be mentioned a man named George Washington or Abraham Lincoln. But every Generic nation set in the generic Twentieth Century will have a written constitutions. And they will always used the Bald Eagle as their symbol -- but let us never mention That Nation, lest someone in 4008 be offended.

Well, all you guys writing Generic Middle Ages, I hate to tell you, but --- only Christendom ever had a "Middle Ages" between the fall of Christian Rome and the rise of the powerful Christian Monarchies of the Renaissance. And even those monarchs were not like the leaders of other nations in Asia and Africa, because their theory of kingship was based on the sacrament of coronation, the anointing, and based on the idea of a law above even the law of kings, and other ideas that have their roots in Charlemagne, and in the Holy Roman Empire. A sultan is not a king, Rex, regis, and neither is a Tenno nor a Shogun. Kingship is different. There are periods in history when other empires rose and fell, but no example in any history of any continent, except Europe, of an empire that fell, whose culture and religion was preserved by the barbarians who overran it, and who erected the same empire again over and over: Caesar was born again as Czars and Kaisers: and the Senate was seated again in the Capitol, except on Capitol hill in Washington, not on the Capitoline hill in Rome. The Roman Empire did not fully die until Napoleon.The Roman Republic lives on in America, even if Roman virtue, once ours, is now gone.

So -- Dragon Crystal Jesus is a fine word to capture my annoyance with That Religion Whom None Dare Name, and they worship a lawful-good henotheistic deity named Generic.