How common was cannibalism in Christian Europe until the 17th century?

There have been a number of societies which normally - and not just in case of famine and starvation - practiced cannibalism for food, and others which practiced ritual forms of cannibalism. But probably not nearly as many as is popularly imagined.

I am not certain that Christian Europeans needed to accuse the societies they encountered elsewhere of cannibalism and other atrocities to justify invading and conquering them.

Early renaissance conquerors had a medieval view of life, and believed that non Christians worshipped false gods, and thus devils, and thus were devil worshippers, and thus were evil, regardlessof how advance and civilized or primitive their technology was, or of how gentle or cruel their practices were.

Christian conquerors claimed that if they forced non Christian people to convert to Christianity they would be saving their souls and the souls of their descendants, and that whatever price the Christian conquerors charged for such a great service, even sometimes enslavement, was cheap compared to the benefit they brought.

In the 19th century when European societies became far more technologically and scientifically and industrially advanced than even the most advanced and ancient non European civilizations, and when European societies became more humanitarian than many non European societies - though still quite brutal compared to present day societies, to say nothing of hypothetical humane future societies - the Europeans stressed more and more the backwardness of other societies and their cruel customs as justifications for conquering them.

You wrote:

Other cases that shocked me include the killing and eating of Johan de Witt by a mob and the torture of György Dózsa.

According to Cassius Dio in his Roman History, Book 59, Section 29:

For Chaerea and Sabinus, pained as they were by the disgraceful proceedings, nevertheless restrained themselves for five days. 6 But when Gaius himself wished to dance and act a tragedy and for this purpose announced three more days of the entertainment, the followers of Chaerea could endure it no longer, but waiting merely till he went out of the theatre to see the boys of exalted birth whom he had summoned from Greece and Ionia ostensibly to sing the hymn composed in his honour, they intercepted him in a narrow passage and killed him. 7 When he had fallen, none of the men present kept hands off him, but all fell to stabbing him savagely, even though he was dead; and some even tasted of his flesh. His wife and daughter were also promptly slain

https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Cassius_Dio/59*.html[1]

So Cassius Dio wrote that some of the flesh of Gaius Caesar Augustus Germanicus, nicknamed caligula, was eaten by his slayers.

Walter VI, Count of Brienne, and exiled Duke of Athens, became Lord of Florence for a few months in 1342 to 43 but was driven out by a revolution.

According to Niccolo Machiavelli, History of Florence and the Affairs of Italy, book II, Chapter VIII:

Many had come to Florence in defense of the people; among whom were a party from Sienna, with six ambassadors, men of high consideration in their own country. These endeavored to bring the people and the duke to terms; but the former refused to listen to any whatever, unless Guglielmo da Scesi and his son, with Cerrettieri Bisdomini, were first given up to them. The duke would not consent to this; but being threatened by those who were shut up with him, he was forced to comply. The rage of men is certainly always found greater, and their revenge more furious upon the recovery of liberty, than when it has only been defended. Guglielmo and his son were placed among the thousands of their enemies, and the latter was not yet eighteen years old; neither his beauty, his innocence, nor his youth, could save him from the fury of the multitude; but both were instantly slain. Those who could not wound them while alive, wounded them after they were dead; and not satisfied with tearing them to pieces, they hewed their bodies with swords, tore them with their hands, and even with their teeth. And that every sense might be satiated with vengeance, having first heard their moans, seen their wounds, and touched their lacerated bodies, they wished even the stomach to be satisfied, that having glutted the external senses, the one within might also have its share. This rabid fury, however hurtful to the father and son, was favorable to Cerrettieri; for the multitude, wearied with their cruelty toward the former, quite forgot him, so that he, not being asked for, remained in the palace, and during night was conveyed safely away by his friends.

https://www.gutenberg.org/files/2464/2464-h/2464-h.htm#link2H_4_0012[2]

So this makes three cases of alleged cannibalism against political foes in Europe, in AD 41, 1343, and 1672, over a span of 1,631 years, that I have read of. Are those the only such cases that happened, or were they only a small minority of a large number of cases of cannibalism that actually occurred in Europe in that era?

It is said that when former King of France Louis XVI was guillotined in 1793, some onlookers in the crowd dipped their handkerchiefs in his blood.

And this appears to be confirmed by genetic tests on the alleged blood of Louis XVI stored in a gourd.

https://www.livescience.com/25914-squash-holds-king-louis-xvi-blood.html[3]

And somewhere on the internet I read that many who soaked up Louis XVI's blood did so according to the superstition that royal blood had magical healing properties. And thus they would have had medicinal uses for the blood. If drinking someone's blood counts as medicinal cannibalism, then there could have been cases of cannibalism with the blood of louis XVI. And possibly with the blood of other beheaded persons.

And there is also this statement:

.. Conradin (1252-1268), their son, was Duke of Swabia and King of Sicily in 1266. However, his kingship was opposed by Charles of Anjou who had the support of Pope Urban IV. Conrad took up arms to keep his throne, but was defeated and captured. He was beheaded on October 28, 1268. His body was dismembered and pieces of his flesh were passed around to the watching crowd, at age sixteen (16).

https://www.angelfire.com/mi4/polcrt/FrederickIIHRE.html[4]

And I think that this is probably just a story which appeared, perhaps centuries ago and yet still in the more recent part of the 753 years between 1268 and 2021.

And if pieces of Conradin's flesh were passed around to the crowd, what was the purpose? To handle the pieces and then pass them on? To keep them as souveieers? To eat?

Very uncommon due to religious reasons

One thing that is definitely clear from the Bible is the fact that cannibalism was regarded with disdain. In fact, as explained in this article, it was more then that : God would punish extremely wicked people with cannibalism. One example of this is Leviticus 26:28-29

28 then in my anger I will be hostile toward you, and I myself will punish you for your sins seven times over. 29 You will eat the flesh of your sons and the flesh of your daughters.

Another, which also deals with necessity during the siege is Deuteronomy 28:53-57

53 Because of the suffering your enemy will inflict on you during the siege, you will eat the fruit of the womb, the flesh of the sons and daughters the Lord your God has given you. 54 Even the most gentle and sensitive man among you will have no compassion on his own brother or the wife he loves or his surviving children, 55 and he will not give to one of them any of the flesh of his children that he is eating. It will be all he has left because of the suffering your enemy will inflict on you during the siege of all your cities. 56 The most gentle and sensitive woman among you—so sensitive and gentle that she would not venture to touch the ground with the sole of her foot—will begrudge the husband she loves and her own son or daughter 57 the afterbirth from her womb and the children she bears. For in her dire need she intends to eat them secretly because of the suffering your enemy will inflict on you during the siege of your cities.

From these two examples (and there are many more), no Christian Church could tolerate cannibalism, and would have to consider it as a grave sin. As such, cannibalism could not be a part of religious ritual or state sanctioned ceremony (unlike for example among Aztecs) .

What could then cannibalism be ? Only something that would be done in a moment of complete collapse of society and its laws. Of course, famine is such a moment, and greater the famine, greater were incidents. However, idea that human flesh was sold openly (and peacefully) on town markets sounds ludicrous. Each town would have local feudal lord and church authorities, not to mention trade guilds and such. It seems quite unlikely they would allow such trade under any circumstance if they still had authority. Imagine how would it look like if someone said : "You could buy human flesh in a fief of Baron X" . What is possible is situation of temporary anarchy and loss of any kind of government. In such circumstances, if local feudal lord and priest could not keep order, anything was possible.

This loss of law and authority is also the reason for cannibalism in Crusader Wars. This time we have a starving unruly army, and prisoners considered heathens (because they are Muslim and of different race). Again, this is clear case where commanders of the army choose to turn blind eye because they know they cannot establish discipline, knowing they could be attacked and killed themselves. Note that story about Richard the Lionheart seems like just that : a story. There could be a grain of truth, considering that crusaders were indeed sometimes starving. But even if the crusader king was forced to eat human meat, it is unlikely he would boast about it, knowing that whole ordeal could be used against him. So it is quite possible that the whole fable is just propaganda against Richard, or a way to dehumanize opponents - Saracens are just pigs.

Then we have cases of Johan de Witt and György Dózsa. Both men were powerful in their time, but also had bitter enemies. In case of Dózsa, he and his rebels were going against established feudal order, so he was punished in a very cruel and unusual manner, but so were his followers - remember that eating human flesh is considered as punishment for sins. Him being eaten, and them being forced to become cannibals only established them as outlaws. Case of de Witt is not so clear, but looks like hatred against him was so great that some of his enemies abandoned all pretense of civilization when they finally got him. Note that manner of his death is remembered even today, so this could not be considered as something that happened everyday.

Finally, we have cases of so called pharmaceutical cannibalism. Unfortunately this practice continues to this day. Samples of aborted baby tissues are used in pharmacy even today. Of course, they are not used as a meal, and similarly remains of the mummies were not used in this manner in past ages. Instead, you would have a powder, a drink, cream etc ... i.e. something that would not look like human body part. In any case, practice could not be widespread. Mummies, even fake ones, were not so common . At best, this could be fringe entertainment in higher circles, not something that would be done openly in the streets.

Outside of literally starving to death and having no other option, as it happened as recently as the Siege of Leningrad in WWII, the practice of eating corpses was as rare as it is today in the Western Civilization, reserved to a few crazy people like Armin Meiwes, the most notorious still living cannibal. To the point you hear of such tales written on a tone designed to shock or very embelished. Most civilizations that developed a form of writing moved away from cannibalism and human sacrifice eventually (notable exception: the Aztecs).

But in speaking of Christian Europe and the consumption of human flesh, what happened during the Roman era might give us a clue to answer your question.

This article, supported by this article, claims that acts of Cannibalism perpretated by Christians were a form of slander. Given that those romans were also fond of Damnatio-ad-bestias-ing christians, it's not surprising at all that they would spread such myth to suppress the religion. Christians also didn't help themselves, consideren bread and wine to be the flesh and body of Christ, a purely symbolic statement meaning that if you live in servitude to God so God can provide you with that Jesus meat (not literally), he'll grant you ethernal life (also not literally).

If accusations of Cannibalism were used as slander by Romans, the only logical conclusion you can reach from that is that the Romans themselves abhorred the practice. In fact, all forms of human sacrifice were outlawed in Rome still in it's republic years, in 97 b.C. They still drank the blood of slain gladiators though, believing they'll get the strenght of the defeated, which kinda doesn't make sense to become as strong as a loser, amiright?

Back to Christianity, it was heavily shaped by roman culture and society while it developed and spread throughout the Empire. Some scholars believe that certain christian dogmas purposefully developed to challenge and dismantle Roman power, however in therms of Cannibalism, Christianity also developed from Judaism, which according to this not very good article also abhorred the practice 3000 years before Christendom.

Fast forward to the late Renaissance, From this article, the question is more like "What sort of flesh should you eat?" rather than "Should you eat human flesh?". Concoctions made out of human remains were rather commonly, the article claims, used by european people. This was due to, as the article puts it, "magical thinking" of "like cures like" and the not very widespread practice (yet) of the Scientific Method. So they thought consuming powdered skull cured headaches and sometimes the headache really did go away, presumably after drinking some water with your brainbox powder because dehydration is the easiest way to get a headache. "Hey, it Worked!" they thought.

But they did consume it exclusively for medicine, though withouth any scientific basis. That last article also states that Mummy was also sold in germany as late as 1908. Given that these procedures don't always work and when they do, they aren't a permanent solution most of the time, could we say that blood transfusions and organ transplants are also a form of cannibalism?