I should start by noting that I have neither worked on nor read a great deal about representations of Islam in the medieval Europe. What I can do is discuss the two Middle English poems that you mention here, William Langland’s Piers Plowman (c.1370–1390) and John Lydgate’s Fall of Princes (1431–1438), illustrate their discussions of Muhammad as a Christian, and try and put that into the context of late medieval English religious politics. Ideally, someone else might be able to re-contextualise what I can provide from a point of view that’s more fully conscious of the history of Christian-Muslim relations.

That said, let’s start with Langland. Piers Plowman exists in three circulated versions, the A, B and C Texts. I’ll just talk about the B Text here as it is the most widely edited and studied – the A Text is basically a short draft, and the C Text is a canny political revision (I’d love to see if anything changes in the Muhammad passage in the C Text, but unfortunately I don’t own a copy). I’ll be quoting here from Schmidt’s edition of the B Text in which the passage is Passus XV 392–415 (pp. 265–6), but another edition is available online as the Piers Plowman Electronic Archive (here Bx.15.409–32). In the passage Langland explains why there are similarities between Christian and Islamic beliefs, recounts a miracle with a dove and the conversion of Syrians to Islam, and then uses this as a metaphor for corrupt Christian preachers (which I’ll discuss further below). The reference to Muhammad as Christian is part of a long discussion between the dreaming narrator and a personified Conscience, and goes like this:

For Sarsens han somwhat semynge to oure bileve,

For thei love and bileve in o Lede almyghty,

And we, lered and lewed, bileveth in oon God―

Cristene and uncristene on oon God bileveth.

Ac oon Makometh, a man, in mysbileve

Broughte Sarsens of Surree – and see in what manere.

This Makometh was Cristene man, and for he moste noght ben a pope,

Into Surrie he soughte, and thorugh hise sotile wittes

Daunted a dowve, and day and nyght hire fedde.

For Saracens have some resemblance to our beliefs, for they love and believe in one Lord almighty and we, learned and unlearned, believe in one God; Christian and non-Christian believe in one God. But one man, Muhammad, brought Saracens of Syria into misbelief – and [you shall] know in what way. This Muhammad was a Christian man, and because he could not be a pope, he went into Syria, and through his subtle wits tamed a dove, and fed her day and night.

Thence follows the miracle with the dove (Muhammed hides corn in his ear, so that the dove will appear to whisper the divine word into it as he preaches) and the conversion of the Syrians to Islam. Schmidt’s note (p. 469) on the ultimate source of this passage ascribes it to a twelfth-century life of Muhammad by Hildebert of Lavardin – we now know that this is a misattribution and that the author of the Vita Mahumeti (first quarter of the twelfth century) was Embrico of Mainz. I know nothing at all about this text, but defer to volume 3 of Christian-Muslim Relations. A Bibliographical History (pp. 592–4, handily available on Google Books), which notes that Embrico draws on various Byzantine and Iberian sources and recasts the material with the deliberate aim that “Islam is reduced to an old and particularly corrupt species of heresy – to something that can be easily understood and utterly dismissed”. This seems to account for the details given by Langland in Piers Plowman and (as I shall discuss) the depiction of Muhammad as a heretical ex-Christian and would-be pope (as well as magician, deceiver, etc.) was convenient to Langland’s polemical purposes.

Lydgate’s account of Muhammad in the Fall of Princes might derive, ultimately, from the same source. Lydgate’s book is an adaptation/translation of Laurent de Premierfait’s extremely popular French Des cas des nobles hommes et femmes (1400 and 1409), itself a translation of Boccaccio’s Latin De Casibus Virorum Illustrium (mid fourteenth-century). Dimmock’s Mythologies of the Prophet Muhammad (pp. 38–41, again available online) suggests that the source for the passage in Boccaccio is Vincent of Beauvais’ thirteenth-century Speculum historiale but notes that Lydgate seems to incorporate details from Jacobus de Voragine’s Legenda aurea (c.1260) and Ranulf Higden’s Polychronicon (second quarter of the fourteenth century). It’s quite possible that Langland also had access to some of these sources, and the accordance of many details with Lydgate suggests that Embrico’s earlier Vita might lie behind all of their descriptions of Muhammad.

Unfortunately the best edition of the Fall of Princes is Bergen’s very old one, but you can read the relevant passage (Book IX, 50–161) here. Lydgate’s Muhammad is not a wannabe-pope but a “fals prophete and a magicien” who gains prestige in Khorasan through a mixture of smart talk and magic tricks. Lydgate also recounts false miracles (the doves, and the hanging of milk and honey from the horns of a bull) and discusses some Islamic practices (Friday prayer, the prohibition of alcohol). Interestingly he does not call Muhammad a Christian, but notes that he “to Iewes & Cristene sondry tymes sent, / Lerned the Olde and Newe Testament” (often wrote to Jews and Christians [and] learned the Old and New Testament). Lydgate doesn’t consider Muhammad’s Christian learning as constituting a Christian spiritual identity, but as the source material for convincing deception – he ascribes fitting episodes (“hadde a siknesse, fil ofte sithes doun” had an illness, fell down very often) to visions of Gabriel, and models his miracles after those of Moses.

Obviously both Langland’s representation of Muhammad as a heretical ex-Christian and aspirant pope and Lydgate’s depiction of someone who uses Christian learning for personal gain are both rooted in contemporary negative attitudes to Islam in late medieval England and precedents in the medieval European literary tradition more generally. As I’ve said, this is a subject I only know about in quite general terms and hopefully someone else can add to the discussion there.

However, there is an additional context for the Muhammad passages in both Piers Plowman and The Fall of Princes that is immediate and specific to the late medieval English cultural landscape, namely that of contemporary discourse on Wycliffite/Lollard heresy – and this I can discuss in a little more detail. The joint pressures of the Western Schism (1378–1417), the incendiary theological writings of the Oxford scholar John Wyclif (d. 1384) and the events surrounding the ascension of Henry IV (1399) led to a delicate and occasionally quite violent religiopolitical climate in late fourteenth- and fifteenth-century England. There’s an awful lot that could be gone into here (the best book on the subject remains Anne Hudson’s Premature Reformation), but the long and short of it is that a loose movement of a quite diverse group of English people began to entertain a great many heterodox ideas: that the clergy was corrupt; that the Bible should be translated into English; that the Eucharist was merely bread and wine; that pilgrimages and religious images were idolatrous; that the Pope was the Antichrist. Etc.

Langland’s Piers Plowman actually interacts with many of these ideas – the B Text from the luxury of a less fraught religious climate, the later C Text with redactions and omissions to evade charges of heresy (although it should be noted that while Langland – at least to my mind – reads more like a learned and conscientious critical thinker than someone actually trying to write heretical polemic). Lydgate, at times a poetic mouthpiece for the Lancastrian dynasty, wrote excoriatingly against Lollard rebellion. The interactions of both these writers and others with English heretical/anti-heretical discourse is discussed at length in Andrew Cole’s Literature and Heresy (not an uncontroversial book, but one I have a lot of time for).

To get back to the Muhammad passages, what I want to emphasise is that late medieval English literary depictions of non-Christian belief are often inflected with the same language used to express anxieties arising from the Wycliffite controversy, and Langland’s and Lydgate’s are no different. References to heresy, to idolatry, to “false” miracles and devotional acts, to unscrupulous things done with textual Christian learning – all resonate with the things these and other writers say about early Lollardy. One can understand why Lydgate would want to depict Muhammad as a divisive non-Christian figure abusing Scriptural learning (as the orthodox position on Lollardy held), and how Langland conversely makes Muhammad into a pseudo-pope whose actions expose similar corruption in the institutional Church. Note what Langland’s Conscience says at the end of the passage:

And siththe Oure Saveour suffred the Sarsens so bigiled

Thorugh a Cristene clerk acorsed in his soule―

Ac for drede of the deeth I dar noght telle truthe,

How Englisshe clerkes a colvere fede that Coveitise highte,

And ben manered after Makometh, that no man useth trouthe.

And thus Our Saviour suffered the Saracens to be so beguiled through a Christian clerk, accursed in his soul. But except for the threat of death I would have dared not tell the truth: how English clerks feed a dove that is called Covetousness, and are mannered after Muhammad, that no man acts according to the truth.

For Langland, the foundation of Islam is an instructional parable in the corruption of an institutional Church that has all the trappings of a Christian faith but lacks any of the virtue.

(some conclusions and a bibliography in my reply)