There is a persistent conception dating from at least the 19th century (as shown in this quote--thanks to Duncan Head for pointing it out to me) that the clergy in medieval Europe were somehow restricted to the use of blunt weapons like clubs, staves, and maces when they were permitted and/or required to fight. The idea further claims that this restriction was a solution created so that the clergy could fight without having to shed their enemies' blood. I don't know how widespread this misconception is outside gaming and fantasy circles influenced by the earlier editions of Dungeons & Dragons; all the same, in this post I will try to show that it is not supported by the available body of evidence about militant European clergy in the Middle Ages, and to present several other modern scholarly (and not-so-scholarly) arguments that have been used to help demolish the idea.
We begin by taking a look at the two cases most frequently used to illustrate the myth. The first and better known is Odo, Bishop of Bayeux--William the Conqueror's half-brother. Odo fought beside William in the Battle of Hastings (1066), and a scene in the Bayeux Tapestry shows him wielding a club amidst a knot of Norman horsemen. However, the Tapestry's narration does not mention any relation between the club and Odo's clerical status, only that he was carrying the club while he was encouraging his "boys" and directing them into the attack. It would also be unwise to take the bishop's battle scene out of the context of the greater narrative, as the very next scene in the Tapestry shows William--not a clergyman--similarly wielding a club as he lifted his helmet to convince his troops that he had not been killed in battle. Most scholars nowadays believe that the club had nothing to do with Odo's clerical background. Rather, its presence in both Odo's and William's hands implied that it was a mark of authority among the Normans, an idea that derives additional support from the possibility that the Normans might have used the wooden club as a conscious Neoclassical allusion to the vine-branch cudgel used by the ancient Romans. Another line of reasoning that leads to the same conclusion is that the Bayeux Tapestry itself was likely done under Odo's patronage at a time when he was eager to show himself off as an equal (or at least a near-equal) to William's social and political prestige. Certainly he appeared almost as often as William did in the Tapestry!
The legends surrounding Turpin (Tilpin), Archbishop of Rheims present many troubles to modern scholars since it is not unlikely that some or all the military exploits ascribed to him were actually the deeds of Milo, one of his predecessors in the archiepiscopal seat. But let us forget the historical Turpin for a while; the person that matters to us is the legendary and literary Turpin, one of the Twelve Peers of Charlemagne. It is true that this Turpin occasionally uses a club or a mace in battle, but on the one hand some of the legends also describe the other (non-clerical) knights fighting with the mace as well, and on the other practically every one of them has Turpin driving his enemies before him with a lance or a sword (especially in tales about the battle at Roncesvalles/Roncevaux Pass, where Turpin is usually described with a lance). There are even several tales that contain no mentions of him wielding a mace or club at all, putting a lance or sword in his hand every time he appears in a martial capacity. So, the legendary Turpin does not make a very good case for the restriction of militant clergy to blunt weapons; if anything, the literary treatments of him tend to weaken rather than support the idea.
Those were just two men out of the many European clergymen who served in a military capacity during the Middle Ages--perhaps just one, considering that recent scholarship has cast doubt upon the identification of Archbishop Turpin with the militant bishop in Charlemagne's retinue. So what about the others? Well, this is where the misconception falls down, actually. Medieval annals are full of wars and battles where militant clergymen figured prominently. The vast majority of the clergymen mentioned in these military affairs naturally came from the higher ranks of the clerical hierarchy--bishops, archbishops, cardinals, and popes. These men generally came from the nobility or at least from the gentry, and nowhere does this show more obviously than in the way that they fought like other nobles and knights of comparable social status. After all, they would have had largely the same military training as other men-at-arms; why would their preferences in armament be any different? A good example is available in a 13th-century illustration to Einhard's famous 9th-century biography of Charlemagne. The upper panel depicts a man wearing a helmet crest in the conspicuous shape of a bishop's mitre--almost certainly indicating that he's a bishop, and perhaps even Turpin--while wielding a lance. The lower panel also has a man wearing a very similar crest (probably the same bishop, only in a different scene) and resting a drawn sword on his armored shoulder. Regardless of whatever else we may say about medieval illustrators, they definitely didn't feel there was anything incongruous in the idea of a bishop wielding pointed or edged weapons!
The other important class of militant clergy is, of course, the monastic Military Orders. The Knights Templar are already a household name among most people who discuss medieval European history with any frequency; the Knights Hospitaler and Teutonic Knights are only marginally less well known, while other orders like the Knights of Calatrava were definitely a significant presence in the European military constellation of their time even if they are not as popular in the minds of a modern audience. It's easy to see the most obvious similarity between all these Orders; they all have the word "Knight" in their popular names, and usually in their full official names as well. (For example, the official title of the Knights Templar in English is the "Poor Knights of Christ.") As the modern public already knows without any need of academic prodding, the ubiquity of this word directly translates to their military equipment and behavior--both of which were just like any other men-at-arms in their time, including the use of lances and swords and daggers and crossbows and other kinds of blood-shedding weapons.
Now, there were examples of lower-class men from the religious professions (both clerical and monastic) who fought with blunt weapons like staffs and clubs. But there's still no historical support whatsoever for the idea that this was because they didn't want to shed blood. The priests and monks who wielded clubs usually did so because they couldn't afford anything better--that is, their weaponry is a product of their low social status rather than their religious professions. Those who fought with staffs, on the other hand, generally did so out of respect to a long-standing tradition for the use of staffs in their local area, especially in England. (It is interesting to note that Thomas a Becket--the Archbishop of Canterbury known for being assasinated by the supporters of King Henry II--is included in this latter category, although I don't remember whether the mention of him being reprimanded by the law for his propensity to fight with the staff comes from the period before or after his accession into the clergy.) And even in this case there is ample evidence that the men of religion were perfectly willing to fight with edged and/or pointed weapons when they could. Remember Friar Tuck of Robin Hood fame, and his skill with sword and buckler? Even outside the realm of fiction, the illustrations in the German sword-and-buckler manual known as the Manuscript I.33 clearly include monks (and a woman!) demonstrating the techniques described in the manuscript's texts.
Anna Comnena--a Byzantine princess--wrote this about a priest in the Crusading armies that passed through Byzantine lands on their way to Jerusalem:
A certain Latin priest who happened to be standing in the stem with twelve other fighting men, saw this, and let fly several arrows against Marianus. Not even then did Marianus surrender, but fought fiercely himself and encouraged his men to do the same, so that three times over the men with the priest had to be replaced, as they were wounded and sore pressed. The priest himself, however, although he had received many blows, and was streaming with his own blood, remained quite fearless. For the rules concerning priests are not the same among the Latins as they are with us ; For we are given the command by the canonical laws and the teaching of the Gospel, " Touch not, taste not, handle not! For thou art consecrated." Whereas the Latin barbarian will simultaneously handle divine things, and wear his shield on his left arm, and hold his spear in his right hand, and at one and the same time he communicates the body and blood of God, and looks murderously and becomes 'a man of blood,' as it says in the psalm of David. For this barbarian race is no less devoted to sacred things than it is to war. And so this man of violence rather than priest, wore his priestly garb at the same time that he handled the oar and had an eye equally to naval or land warfare, fighting simultaneously with the sea and with men.
Even accounting for dear Anna's biases against the Catholic Crusaders (she was a Greek Orthodox), we can't help noticing that she made no references to the idea that the clergy were restricted to blunt weapons; the priests mentioned in her chronicles either didn't fight at all or fought with any and all weapons they could use, including blood-shedding spears and arrows.
The mace is the weapon most frequently associated with fantasy depictions of militant clergy. In a way, this is not inappropriate, because the militant clergy of the higher orders (that is, those with enough wealth or social status to be men-at-arms) did carry maces. The proliferation of this weapon increased steadily after the introduction of reinforced surcoats and coats-of-plates that provided some form of rigid defense to reinforce flexible coats of mail, and even more after the introduction of plate armor. But the increase did not happen only among the militant clergy--it happened across the board among practically all men-at-arms. There isn't any evidence to support any idea that the militant clergy took on these concussion weapons more eagerly than did their non-clerical brethren.
In short, the militant clergy increasingly adopted the mace from the 13th century onwards not because of any special religious compunction, but because of the same reasons that motivated other, non-clerical knights to take up the weapon--namely, the need to overcome heavily-armored opponents. It is also worth mentioning that, for both clerical and non-clerical men-at-arms, the mace supplemented the sword and the lance rather than entirely replacing either of them.
All this silliness about militant clerics and blunt weapons would probably never have happened if people did not fall for the idea that "blunt weapons don't shed blood." It's easy to demonstrate that this idea is plainly not true. Anybody can go around and research the effects of a blunt object on human flesh without much difficulty. Search the Web or the local library's newspaper archives about fist-fights, or about brawls that involved clubs and/or batons; look for information about "friendly" fights involving blunted weapons, especially the show fights between Dusack/Messer-wielding opponents that became quite popular in late Renaissance germany; watch a boxing or MMA (mixed martial arts) match; or failing that, go pick a target that wouldn't fight back and slam a fist, a crowbar, or a wooden plank onto his/her head. More sensitive readers might want to replace the last step with knocking their own head on a table or a door lintel. Any of these methods would quickly reveal one painful fact: the human body, especially the head, has many blood vessels near the skin, and a sufficiently strong blow from a blunt object can tear the skin enough to cause bleeding from these peripheral blood vessels. So there's no need for a sharpened edge or an acute point if we want to cause bleeding in the opponent. We just have to hit hard enough on a vulnerable spot--preferably the head--and the blood will flow. Repeat: whack them with a stick, and the blood will flow.
So now let's get into a bit of mental extrapolation. If a fist or a club is already perfectly capable of breaking the victim's skin, how bad would the poor bastard's plight be if he/she was struck by the concentrated mass of a mace's head? Or, to make it even gorier, why not use the massive, narrow head of a warhammer, which doesn't need any sharp points to puncture an unarmored human's skull (though the hammer often has one or several anyway)?
There. I guess I've given enough satisfaction to my psychopathic side. Now I'll have to go and find a world for my megalomaniac self to conquer....