“You mustn’t let facts get in the way of a good story,” Boris Johnson was reported to have once told the French journalist Jean Quatremer in the early 1990s. It is a claim that defined much of his journalistic career and also appears to shape his pronouncements on the Muslim faith. In an essay written by Johnson in 2007 and unearthed by the Guardian this week, he claims that the Muslim world is “centuries behind” the west, because of a “fatal religious conservatism” that prevented the development of liberal capitalism and democracy. According to Johnson “virtually every global flashpoint you can think of – from Bosnia to Palestine to Iraq and Kashmir” is defined by “some sense of Muslim grievance”. Echoing his hero Winston Churchill’s view that there was “no stronger retrograde force” than Islam, Johnson believes “that the real problem with the Islamic world is Islam”.

Johnson has been here before, with his attacks last year on the Muslim faith as “bizarre and unattractive”, and likening women in burqas to “letterboxes” and “bank robbers”. This clearly played well with the Tory grassroots: a recent poll of party members found that 56% believe Islam is “a threat” to the “British way of life” (whatever that is). But Johnson’s 2007 essay – an appendix to a later edition of his book praising the Roman empire – reveals a level of historical ignorance shocking even for such a political opportunist.

He claims Byzantine Constantinople “kept the candle of learning alight for a thousand years”, while the Ottomans failed to develop printing presses in the city “until the middle of the 19th century”. Wrong. Byzantine rule had gone backwards for generations prior to its fall to the Ottoman sultan Mehmed II in 1453, who repopulated the ruined city with Jews and Christians to help build one of the most sophisticated and cosmopolitan centres of its time, courted for its commercial power by Venice and a magnet for Renaissance Italian scholars and artists (Leonardo even proposed a design for a bridge across the Golden Horn for the sultan in 1502). The city’s first officially recognised printing press opened in 1727, not because of previous objections by zealous mullahs but because of the Islamic handwritten calligraphic tradition that regarded words as art – something print struggled to reproduce.

Sultan Mehmed II transformed Constantinople into ‘one of the most sophisticated and cosmopolitan centres of its time’. Photograph: Leon Neal/AFP/Getty Images

Johnson argues there is nothing like Michelangelo’s Sistine Chapel in the Muslim world “because it is beyond the technical accomplishment of Islamic art” and “theologically offensive to Islam”. Wrong. He might like to know that scholars now believe Michelangelo took inspiration for designing St Peter’s from the imperial mosques designed by the Ottoman architect Sinan, who also influenced the other great Italian architect, Palladio. Johnson’s attack on Islamic aniconism – the rejection of figurative images – betrays a profound ignorance of both Islamic calligraphy and the differences between Sunni and Shia traditions in representing figures. But then there’s little sense that he even grasps the differences between the two Islamic denominations as he collapses the diversity of what he calls the Islamic world into one angry, ignorant monolith.

As with so much of Johnson’s posturing, none of this is new. His argument is a clumsy rehash of the orientalist scholar Bernard Lewis’s infamous essay, What Went Wrong, published shortly after 9/11, which argued that the “mighty civilisation” of medieval Islam was subsequently reduced to aping the west because of a self-induced lack of political freedom, and led to him advising George W Bush’s administration on the invasion of Iraq in the name of freedom (presumably the fact that it didn’t go too well is also the fault of Islam). Lewis was an academic ideologue with some knowledge of Islam, however much he compromised it to suit his political agenda. Johnson parrots Lewis’s arguments with absolutely no historical understanding whatsoever.

To some extent, his position is understandable: the tradition of Greco-Roman study at Oxford that produced the man we all know has always quietly assumed the “East”, from the Persian empire to the rise of Islam, was its backward, despotic antithesis. And hardly anyone within that field studies Arab or early Islamic history, or bothers learning Arabic.

So the myths and prejudices harden into facts. There is no awareness of the life of Muhammad, a merchant outside the Meccan trading elite, and the early history of the Qur’an. The only Qur’anic passages written in the prophet’s lifetime concerned commerce, representing middling merchants, traders and the poor. So much for a religion inimical to capitalism.

There is no space in Johnson’s rhetoric for the scientific and cultural achievements of medieval Islam. Nor is there any acknowledgment that the “fatal religious conservatism” is primarily down to the influence of Wahhabism, the puritanical doctrine founded in the 18th century that is now the official state religion of Saudi Arabia, which condemns millions of Muslims – including Shias – as apostates and has inspired terrorist organisations such as Isis. But Johnson would never acknowledge any of this because of the fear of jeopardising future trade in arms, oil and aircraft with the Saudis. There is no awareness of the so-called Islamic Enlightenment, the subject of Christopher Bellaigue’s book, cataloguing reformers in Egypt, Turkey and Iran who, from the 19th century, embraced industrialisation and struggled in support of political constitutionalism, scientific inquiry and gender equality.

None of this would concern Johnson, because his anti-Islam statements only shore up his political base in the short term, regardless of historical reality. But “speaking your mind” based on proven ignorance is no way to engage in meaningful political dialogue with a quarter of the world’s population.

Let’s see how successful that is as a strategy for Johnson over the coming years.

• Jerry Brotton is professor of Renaissance studies at Queen Mary University of London