This question poses an interesting challenge that can shed light on how historians think about evidence, and how we often take different approaches to finding an 'answer' to our questions.

To start with, most of the physical features of the modern image of the Devil developed through the medieval period as the Devil (Satan) became more real to Christians, as he began to stalk the earth in the minds of ecclesiastics. This process of materializing the devil begins in the latter part of the early middle ages, late Carolingian. These physical features given by ecclesiastics are not represented in the Bible.

The problem medievalists face is that documentation between late antiquity through to the high middle ages is thin. How did the Devil get the form he was given, why was he given his specific look? Historians must turn to archaeology, architecture, arts, visual evidence where text is absent or mute. For some medievalists, this means a turn to formalism. Here is what medievalist Norman Cantor has to say about formalism:

A definition of formalism in medieval studies might be the way of interpreting literature or art that stresses the heavy or exclusive dominance of traditional standard images or motifs, perpetual coded formulas of representation and description. The traditional, standardized images and motifs are privileged and centered in this view of medieval visual and literary art, while individual creativity and original discovery are marginalized or excluded altogether. Formalists regard medieval literature and art as overwhelmingly dominated by traditional sets of images and themes and individual creativity in literature and art as rare. [Cantor, Inventing the Middle Ages, William Morrow:1991,p162]

So, for some medievalists, this methodology privileges the persistence of forms inherited over time:

Literary and artistic styles and genres did change, but not the thematic content of ideas in medieval art and literature. Ideas, themes, motifs followed traditional formularies. [p163]

Cantor shrewdly see that there is some foundation to this approach, but also sees it as ideological:

Iconology and topology also speak to the conservative continuity and enduring unity of higher medieval culture. The great preponderance of images and motifs was inherited from Greco-Roman classical art and literature or from the thought world of the Church Fathers [..] which in turn was a product of the interaction of biblical ideas with the classical traditions.

What does all have to do with the devil and his pitchfork?

 

The Devil's Trident

Here is one of the first representations we find of him with pitchfork, on Muiredach's High Cross at the Irish monastery of Monasterboice. The carving is dated to the 10th century. On the right arm it depicts the devil, pitchfork in hand, herding souls away from Jesus at the center, corralling souls off to hell to the blast of a demon's trumpet like some parade in our worst nightmares. Or is it a trident the Devil uses to corral the souls?

The historian Jeffrey Burton Russell did a lot of important work on demonology of the middle ages, from heresy and witchcraft to the history of the devil. He contributed tremendously to understanding the development of the devil in the minds of Latin Christendom and how that in turn affected persecution. His writing is often beautiful, lyrical and convincing with his immense erudition. In some important respects, though, he leaned on formalism, and it shows in his totalizing rationalizations of the origins of witches, witchcraft, and the devil. Here he writes about the devil and his pitchfork:

Three of his characteristics have origins other than the bestial. Wings are an ancient symbol of divine power found on the shoulders of many Mesopotamian deities, and from Mesopotamia they passed over onto the shoulders of the Hebrew cherubim and seraphim. Ahura Mazda in Iran was represented borne aloft by mighty wings. Hermes, the messenger of the god, wore wings upon his ankles or legs. Horns to are ancient symbols of power and fertility. The Devils "pitchfork" derives in part from the ancient trident, such as that carrier by Poseidon, which symbolizes threefold power over earth, air, and sea, in part from symbols of death (such as the mallet of Charun), and in part from the instruments used in hell for the torment of the damned. [Russell, The Devil: Perceptions of Evil from Antiquity to Primitive Christianity, Cornell:1977, p254]

Well, he's just about the only one who has written much of anything on the devil's pitchfork and so it gets repeated and becomes true. Even Russell putting "pitchfork" in quotations sets us up for doubt, that the pitchfork isn't actually a pitchfork. Russell has on his very interesting agenda a rooting of Christian beliefs in traditions that came before it. Accordingly, we should see that the "pitchfork" is an inheritance of a Poseidon's classical, 1000 year old Trident. Following the logic of formalist medievalism, and not entirely without good argument, this image crossed 1000 years from Greece, maybe India, to Ireland and wound up as the image a carver would use in their depiction of the devil. Russell would likely support his arguments with the tremendous storehouse of history he kept in his head to remind us of all the other inheritances that can truly be documented.

There is safety in tradition, in formalism. It compares physical evidence, things we can see and touch. And it provides an answer no matter how great (and often inexplicable) a gap. Whether that answer is right is another matter, and one that I want to challenge.

The problem is that nowhere is the link across 1000 years gap positively affirmed. Not in the bible, not in the writings of Church fathers. No early transitional iconography nor mention of trident in ecclesiastical writings. It develops in iconography of the latter end of the early middle ages, during the period where the devil starts to take his place in the material world. During this time he moves from invisible instigator to walking among us. I wrote a bit about another aspect of the Devil's materialization in the early middle ages in another post on the devil and his suit.

If the devil is to appear in the real world, an innovation in Latin Christendom, than why wouldn't the real world inspire his image?

 

The Devil's Pitchfork

The imagery of the war between God and the Devil as fought on earth was created by ecclesiastics, and principally by monastics. It should be familiar story to most that these monastics lived in rural surroundings. There wasn't much of life in the early middle ages Europe that wasn't dominated by agriculture. Abbeys were situated on cleared land, or land to be cleared.

The early medievals inherited the light scratch-plow from the Romans. But medievals in northern Europe, with heavier, clay soils, innovated the moldboard plow to turn deeper farrows. And so it is that the Romans had a two-pronged pitchfork, but it was smaller, for light work. It appears, in the west, that the three-plus pronged pitchfork developed in the early middle ages. (The trident was used by Romans for fishing, but it does not appear to have persisted past antiquity, nor made its way to the north.)

Farming communities grew up around these abbeys, against their walls. The monastic's daily life would have been suffused with the sights, sounds, smells of rural life, of peasantry. The rhythms of scything, stacking, and moving hay. Of mucking barns. Pitchforks slung over shoulder, propped against walls on breaks, or swinging in hours of constant motion.

These monastics were some of our only witnesses to war in the medieval country side. We know that war was not fought by sword alone, and indeed the peasant with pitchfork was an effective threat to the mounted warrior.

So, if the monastics are now thinking about the devil come to earth, to obtain not just the soul but now the very bodies of people, to be swept, shepherded, cajoled, into the mouth of hell to live a physical pain, when that monastic is searching for imagery with which to arm his rebelling demons, why would he look further than the fields of his monastery, fields filled with the hard working Christian souls tending the fields and barns with the most common of implements?

The Devil's pitchfork appears in the second half of the early middle ages, somewhere after 800 CE. He appears by the hands of ecclesiastical writers and artists: monks and bishops. Did those monks and bishops draw their inspiration from Greece and Rome of a 1000 years before? Or from looking on peasant flock who the monks were just then beginning to worry would be shepherded away by some physical, stalking Satan, prodded and poked from salvation to the gates of hell like some demonic inversion of John the Baptist's vision of salvation by Christ:

His winnowing fork is in his hand, and he will clear his threshing floor, gathering his wheat into the barn and burning up the chaff with unquenchable fire. [Matthew 3:12, Luke 3:17]

-I'm still a student but I feel I can actually contribute to this.

One thing that I've learned about the portrayal of good and evil (this has come from sources such as reading Nietzsche, to examining the Catholic Church before the Protestant Reformation, to even Ancient Greece and Rome) is what is associated with "good" and "evil"

"Good" is mainly associated with the rich- they are "pure", they are clean of the stench that the poor have because the poor must work, they do not have to bother themselves with using tools such as pitchforks, and they also set the viewpoint for good and evil- or that they determine what and who is good and bad. This can be seen easier in societies where one of, of the main upper social classes are composed of the clergy. In Medieval Europe, it was the clergy. In India, the Brahmans, in South/Latin America (before colonization) the tribal leaders of the great empires (Olmecs, Mayans, Incas, Aztecs, etc.) and after, the Spanish.

The ideas of what good and bad looked like were made by those in power- and those in power wanting to look good, pure, etc. also associated the terms with Christianity- or specifically morality. Those who were rich could only be considered good Christians because they were "so obviously blessed" to be born into a rich family, or somehow make the wealth themselves. The bad was everyone else. The poor, the serfs, the slaves, the workers, the foreigners, the conquered peoples, those in exile, who fall out of favor, who are not rich or powerful. They were seen as bad, and so were their habits.

In Ancient Athens (and most of Ancient Greece), it was seen as a disgrace to work for a living. If you didn't have some sort of preexisting wealth, you were still a citizen (assuming you were male, born in Athens, and completed your military service), but you were looked down upon. You had a vote, but you weren't considered as smart as those who didn't have to work for a living. This idea was so en-rooted in Athenian culture, that the philosopher Socrates lived by it- getting all his meals from friends/parties, and living in a small home. Back to the Devil- The poor were associated with bad morality, and the ultimate symbol of bad morality was Hell, Lucifer, the Devil, Satan, whatever you want to call it. Pitchforks were used by farmers, and because farmers had to manually work, and were still poor ?(for the most part) the pitchfork became associated with this sense of bad morality because of its association with what the "good" part of society considered "bad"

Everything has been covered wonderfully- I just want to get this out there (and see if my thought pattern derived from my studies is logical)