During the Middle Ages, Christendom largely viewed Islam as a Christian heresy and Muhammad as a false prophet. Various Western and Byzantine Christian thinkers considered Muhammad to be a perverted, deplorable man, a false prophet, and even the Antichrist, as he was frequently seen in Christendom as a heretic or possessed by demons. Some of them, like Thomas Aquinas, criticized Muhammad's promises of carnal pleasure in the afterlife.
The earliest documented Christian knowledge of Muhammad stems from Byzantine sources, written shortly after Muhammad's death in 632 CE. With the Crusades of the High Middle Ages, and the wars against the Ottoman Empire during the Late Middle Ages, the Christian reception of Muhammad became more polemical, moving from the classification as a heretic to depiction of Muhammad as a servant of Satan or as the Antichrist, who will be eternally suffering tortures in Hell amongst the damned. By the Late Middle Ages, Islam was more typically grouped with Paganism, and Muhammad was viewed as an idolater inspired by the Devil. A more relaxed or benign view of Islam only developed in the modern period, after the Islamic empires ceased to be an acute military threat to Europe (see Orientalism).
The earliest written Christian knowledge of Muhammad stems from Byzantine sources, written shortly after Muhammad's death in 632 CE. In the anti-Jewish polemic the Teaching of Jacob, a dialogue between a recent Christian convert and several Jews, one participant writes that his brother "wrote to [him] saying that a deceiving prophet has appeared amidst the Saracens". Another participant in the Doctrina replies about Muhammad: "He is deceiving. For do prophets come with sword and chariot?, ...[Y]ou will discover nothing true from the said prophet except human bloodshed". Though Muhammad is never called by his name, the author seems to know of his existence and represents both Jews and Christians as viewing him in a negative light. Other contemporary sources, such as the writings of Sophronius of Jerusalem, do not characterize Saracens as having their own prophet or faith, only remarking that the Saracen attacks must be a punishment for Christian sins.
Sebeos, a 7th-century Armenian bishop and historian, wrote shortly after the end of the first Arab civil war concerning Muhammad, what mostly conforms to the Muslim tradition," In that period a certain one of them, a man of the sons of Ishmael named Muhammad, a merchant, became prominent. A sermon about the Way of Truth, supposedly at God's command, was revealed to them, and [Muhammad] taught them to recognize the God of Abraham, especially since he was informed and knowledgeable about Mosaic history. Because the command had come from On High, he ordered them all to assemble together and to unite in faith. Abandoning the reverence of vain things, they turned toward the living God, who had appeared to their father, Abraham. Muhammad legislated that they were not to eat carrion, not to drink wine, not to speak falsehoods, and not to commit adultery. He said: "God promised that country to Abraham and to his son after him, for eternity. And what had been promised was fulfilled during that time when [God] loved Israel. Now, however, you are the sons of Abraham, and God shall fulfill the promise made to Abraham and his son on you. Only love the God of Abraham, and go and take the country which God gave to your father, Abraham. No one can successfully resist you in war, since God is with you."" Sebeos knows Muhammad's name and that he was a merchant by profession. He presents a good summary of Muhammad's preaching and the four prohibitions which are mentioned in the Qur'an.
Knowledge of Muhammad in Medieval Christendom became available after the early expansion of the Islamic religion in the Middle East and North Africa. In the 8th century John of Damascus, a Syrian Eastern Orthodox monk, Christian theologian, and apologist that lived under the Umayyad Caliphate, reported in his heresiological treatise De Haeresibus ("Concerning Heresy") the Islamic denial of Jesus' crucifixion and his alleged substitution on the cross, attributing the origin of these doctrines to Muhammad himself:
And the Jews, having themselves violated the Law, wanted to crucify him, but having arrested him they crucified his shadow. But Christ, it is said, was not crucified, nor did he die; for God took him up to himself because of his love for him. And he [Muhammad] says this, that when Christ went up to heaven God questioned him saying "O Jesus, did you say that 'I am Son of God, and God'?" And Jesus, they say, answered: "Be merciful to me, Lord; you know that I did not say so, nor will I boast that I am your servant; but men who have gone astray wrote that I said this and they said lies concerning me and they have been in error". And although there are included in this scripture many more absurdities worthy of laughter, he insists that this was brought down to him by God.
Later, the Latin translation of De Haeresibus, where he explicitly used the phrase "false prophet" in referring to Muhammad, became known in the Christian West. According to the Encyclopædia Britannica, Christian knowledge of Muhammad's life "was nearly always used abusively". Another influential source was the Epistolae Saraceni ("Letters of a Saracen") written by an Oriental Christian and translated into Latin from Arabic. From the 9th century onwards, highly negative biographies of Muhammad were written in Latin, such as the one by Álvaro of Córdoba proclaiming him the Antichrist. Christendom also gained some knowledge of Muhammad through the Mozarabs of Spain, such as the 9th-century Eulogius of Córdoba, who was one of the Martyrs of Córdoba.
In the 11th century Petrus Alphonsi, a Jew who had converted to Christianity, was another Mozarab source of information on Muhammad. Later during the 12th century Peter the Venerable, who saw Muhammad as the precursor to the Antichrist and the successor of Arius, ordered the translation of the Quran into Latin (Lex Mahumet pseudoprophete) and the collection of information on Muhammad so that Islamic teachings could be refuted by Christian scholars.
During the 13th century, European biographers completed their work on the life of Muhammad in a series of works by scholars such as Peter Pascual, Riccoldo da Monte di Croce, and Ramon Llull in which Muhammad was depicted as an Antichrist while Islam was shown to be a Christian heresy. The fact that Muhammad was unlettered, that he married a wealthy widow, that in his later life he had several wives, that he was involved in several wars, and that he died like an ordinary person in contrast to the Christian belief in the supernatural end of Jesus' earthly life were all arguments used to discredit Muhammad.
Medieval scholars and churchmen held that Islam was the work of Muhammad who in turn was inspired by Satan. Kenneth Setton wrote that Muhammad was frequently calumniated and made a subject of legends taught by preachers as fact. For example, in order to show that Muhammad was the anti-Christ, it was asserted that Muhammad died not in the year 632 but in the year 666 - the number of the beast - in another variation on the theme the number "666" was also used to represent the period of time Muslims would hold sway of the land. A verbal expression of Christian contempt for Islam was expressed in turning his name from Muhammad to Mahound, the "devil incarnate". Others usually confirmed to pious Christians that Muhammad had come to a bad end. According to one version after falling into a drunken stupor he had been eaten by a herd of swine, and this was ascribed as the reason why Muslims proscribed consumption of alcohol and pork. In another account of the alcohol ban, Muhammad learns about the Bible from a Jew and a heretical Arian monk. Muhammad and the monk get drunk and fall asleep. The Jew kills the monk with Muhammad's sword. He then blames Muhammad, who, believing he has committed the crime in a drunken rage, bans alcohol.
Leggenda di Maometto is another example of such a story. In this version, as a child Muhammad was taught the black arts by a heretical Christian villain who escaped imprisonment by the Christian Church by fleeing to the Arabian Peninsula; as an adult he set up a false religion by selectively choosing and perverting texts from the Bible to create Islam. It also ascribed the Muslim holiday of Friday "dies Veneris" (day of Venus), as against the Jewish (Saturday) and the Christian (Sunday), to his followers' depravity as reflected in their multiplicity of wives. A highly negative depiction of Muhammad as a heretic, false prophet, renegade cardinal or founder of a violent religion also found its way into many other works of European literature, such as the chansons de geste, William Langland's Piers Plowman, and John Lydgate's The Fall of the Princes.
The thirteenth century Golden Legend, a best-seller in its day containing a collection of hagiographies, describes "Magumeth (Mahomet, Muhammad)" as "a false prophet and sorcerer", detailing his early life and travels as a merchant through his marriage to the widow, Khadija and goes on to suggest his "visions" came as a result of epileptic seizures and the interventions of a renegade Nestorian monk named Sergius.
In Dante Alighieri's Divine Comedy, Muhammad is in the ninth ditch of Malebolge, the eighth realm, designed for those who have caused schism; specifically, he was placed among the Sowers of Religious Discord. Muhammad is portrayed as split in half, with his entrails hanging out, representing his status as a heresiarch (Canto 28):
This scene is frequently shown in illustrations of the Divine Comedy. Muhammad is represented in a 15th-century fresco Last Judgment by Giovanni da Modena and drawing on Dante, in the San Petronio Basilica in Bologna, as well as in artwork by Salvador Dalí, Auguste Rodin, William Blake, and Gustave Doré.
One common allegation laid against Muhammad was that he was an impostor who, in order to satisfy his ambition and his lust, propagated religious teachings that he knew to be false. Cultural critic and author Edward Said wrote in Orientalism regarding Dante's depiction of Muhammad:
Empirical data about the Orient [...] count for very little [i.e., in Dante's work]; what matters and is decisive is [...] by no means confined to the professional scholar, but rather the common possession of all who have thought about the Orient in the West [...]. What [...] Dante tried to do in the Inferno, is [...] to characterize the Orient as alien and to incorporate it schematically on a theatrical stage whose audience, manager, and actors are [...] only for Europe. Hence the vacillation between the familiar and the alien; Mohammed is always the imposter (familiar, because he pretends to be like the Jesus we know) and always the Oriental (alien, because although he is in some ways "like" Jesus, he is after all not like him).
A more positive interpretation appears in the 13th-century Estoire del Saint Grail, the first book in the vast Matter of Britain, the Lancelot-Grail. In describing the travels of Joseph of Arimathea, keeper of the Holy Grail, the author says that most residents of the Middle East were pagans until the coming of Muhammad, who is shown as a true prophet sent by God to bring Christianity to the region. This mission however failed when Muhammad's pride caused him to alter God's wishes, thereby deceiving his followers. Nevertheless, Muhammad's religion is portrayed as being greatly superior to paganism.
The depiction of Islam in the Travels of Sir John Mandeville is also relatively positive, though with many inaccurate and mythical features. It is said that Muslims are easily converted to Christianity because their beliefs are already so similar in many ways, and that they believe that only the Christian revelation will last until the end of the world. The moral behaviour of Muslims at the time is shown as superior to that of Christians, and as a standing reproach to Christian society.
Depictions of Muhammad in the form of picaresque novel began to appear from the 13th century onward, such as in Alexandre du Pont's Roman de Mahom, the translation of the Mi'raj, the Escala de Mahoma ("The Ladder of Muhammad") by the court physician of Alfonso X of Castile and León and his son.
Medieval European literature often referred to Muslims as "infidels" or "pagans", in sobriquets such as the paynim foe. These depictions such as those in The Song of Roland represent Muslims worshiping Muhammad (spelt e.g. 'Mahom' and 'Mahumet') as a god, and depict them worshiping various deities in the form of "idols", ranging from Apollyon to Lucifer, but ascribing to them a chief deity known as "Termagant".
Conversely, in medieval romances such as the French Arthurian cycle, pagans such as the ancient Britons or the inhabitants of "Sarras" before the conversion of King Evelake, who presumably lived well before the birth of Muhammad, are often described as worshipping the same array of gods and as identical to the imagined (Termagant-worshipping) Muslims in every respect. In the same vein, the definition of "Saracen" in Raymond of Penyafort's Summa de Poenitentia starts by describing the Muslims but ends by including every person who is neither a Christian nor a Jew.
When the Knights Templar were being tried for heresy reference was often made to their worship of a demon Baphomet, which was notable by implication for its similarity to the common rendition of Muhammad's name used by Christian writers of the time, Mahomet. All these and other variations on the theme were all set in the "temper of the times" of what was seen as a Muslim-Christian conflict as Medieval Europe was building a concept of "the great enemy" in the wake of the quickfire success of the early Muslim conquests shortly after the fall of the Western Roman Empire, as well as the lack of real information in the West of the mysterious East.
In the Heldenbuch-Prosa, a prose preface to the manuscript Heldenbuch of Diebolt von Hanowe from 1480, the demon Machmet appears to the mother of the Germanic hero Dietrich and builds "Bern" (Verona) in three days.
Criticism by Christians [...] was voiced soon after the advent of Islam starting with St. John of Damascus in the late seventh century, who wrote of "the false prophet", Muhammad. Rivalry, and often enmity, continued between the European Christian world and the Islamic world [...]. For Christian theologians, the "Other" was the infidel, the Muslim. [...] Theological disputes in Baghdad and Damascus, in the eighth to the tenth century, and in Andalusia up to the fourteenth century led Christian Orthodox and Byzantine theologians and rulers to continue seeing Islam as a threat. In the twelfth century, Peter the Venerable [...] who had the Koran translated into Latin, regarded Islam as a Christian heresy and Muhammad as a sexually self-indulgent and a murderer. [...] However, he called for the conversion, not the extermination, of Muslims. A century later, St. Thomas Aquinas in Summa contra Gentiles accused Muhammad of seducing people by promises of carnal pleasure, uttering truths mingled with many fables and announcing utterly false decisions that had no divine inspiration. Those who followed Muhammad were regarded by Aquinas as brutal, ignorant "beast-like men" and desert wanderers. Through them Muhammad, who asserted he was "sent in the power of arms", forced others to become followers by violence and armed power.-- Michael Curtis, Orientalism and Islam: European Thinkers on Oriental Despotism in the Middle East and India (2009), p. 31, Cambridge University Press, New York, ISBN 978-0521767255.