I'll never forget the moment my ears were opened to Mendelssohn. The Emerson Quartet was playing at the Queen Elizabeth Hall in London, and the last piece on the programme was Mendelssohn's Sixth String Quartet in F minor. I hadn't heard the piece before, and my hopes weren't high. Apart from the preternaturally brilliant Octet, which Mendelssohn composed when he was 16, his chamber music is rarely played in big-name recitals; presumably, I thought, because it wasn't interesting enough. But then the Sixth Quartet began. Mendelssohn wasn't supposed to sound like this: the four movements glowed with a dark fire for 25 minutes. The quartet begins with a tremulous jitter before a fragmented melody appears, but nothing in the music is safe or secure. The wildness of gesture and dissonance shocked me: this was a different universe from the genteel Victorian world I had imagined Mendelssohn was all about.
The Sixth Quartet was Mendelssohn's last major work, composed after his beloved sister Fanny - also a prodigiously talented composer - died in May 1847. Felix would live for just six more months, succumbing to a stroke aged 38. When he died, he was the most famous composer in Europe. His achievements were astonishing: as well as hugely popular orchestral works such as the Violin Concerto, he wrote two of the most popular oratorios of the mid-19th century - Paulus, a big hit in Germany, and Elijah, a staple for British choral societies after its Birmingham premiere in 1846. Mendelssohn also gave the first modern performances of Bach's music, including the Matthew Passion; he raised conducting to the level of an art-form; and he was one of the most accomplished pianists of his generation.
But that's not how he has been remembered. George Bernard Shaw, writing in 1898, accused Mendelssohn of "despicable oratorio-mongering"; for all his fame, he said, he "was not in the foremost rank of great composers". And for today's composers and conductors, Mendelssohn comes a long way down the list. How did this genius composer come to be synonymous with the worst aspects of 19th-century music - its conservatism, nostalgia, sentimentality and superficiality?
I think it's all Wagner's fault. The year after Mendelssohn's death, Wagner wrote a notorious and poisonous essay, Jewishness in Music. In it, he said that Mendelssohn "has shown us that a Jew can possess the richest measure of specific talents, the most refined and varied culture ... without even once through all these advantages being able to bring forth in us that profound, heart-and-soul searching effect we expect from music". In fact, Mendelssohn was the most Christian of Jewish composers: the grandson of Moses Mendelssohn, the Jewish Enlightenment philosopher, Felix was baptised at the age of seven. He was always conscious of his Jewish heritage, refusing his father's suggestion that he be known as "Felix M Bartholdy", and signing himself Felix Mendelssohn Bartholdy instead. Yet he did more to align his music with Protestantism than any other major 19th-century composer, writing scores of Psalm settings, a symphony in honour of the Reformation, and an oratorio, Paulus, that dramatised the most famous conversion from Judaism to Christianity in the Bible.
The reason for Wagner's vitriol was simple: he felt threatened. In the years after his death, Mendelssohn's influence made him the most important figure in German musical culture. Before Wagner could launch his musical and social revolutions, he needed to destroy Mendelssohn.
And that meant turning him into the anti-Wagner. Where the new German music should be strong and ambitious, Mendelssohn's was deemed effeminate and vague; where orchestral performance should be flexible and expressive, his conducting was "flabby and colourless"; where a composer should be part of an emerging German nationalism, Mendelssohn, as a Jew, was "outside the pale of German art-life".
There may have been a more personal element to all this. In 1836, the 23-year-old Wagner sent Mendelssohn - only four years older but already a towering figure - a copy of his C major Symphony. Mendelssohn never replied. Later, Mendelssohn saw the premiere of Wagner's Flying Dutchman. Robert Schumann, who was with Felix, remembers that he was "totally indignant" about it. Had Mendelssohn lived to write the opera he began at the end of his life, the battle for the soul of German music-theatre could have been less one-sided than it became, with the inexorable rise of Wagner's music-dramas.
The tragedy is that Wagner's critique has become - minus most of the racism - the default position when it comes to Mendelssohn. His technical facility, the driving force behind his music, is seen as shallow academicism; the commercial success he enjoyed is seen as proof that he wrote only to please his public.
Added to all this was his status as Queen Victoria and Prince Albert's favourite composer. On the last of his many visits to London, Mendelssohn gave Victoria and Albert a private performance, and the Queen, a passable soprano, sang his and his sister's songs. The wild popularity of Elijah cemented Mendelssohn's position in the British establishment; the luxurious playability of his Songs Without Words for piano made him the musical darling of middle-class living rooms all over the country.
None of this has helped his posthumous cause. Mendelssohn has been seen as a neo-baroque monumentalist and a salon-celebrated sentimentalist - not as heroic as Beethoven or as otherworldly as Schubert, not as wild as Berlioz or as idealistic as Schumann.
Which all means that he has become the biggest blind spot in classical music. Because of the brilliance of its craft, it is very easy to be beguiled by the surfaces of Mendelssohn's music, to imagine that that's all there is to it. But you have to go back to works like the Violin Concerto or the Hebrides Overture and listen to them with open ears, without the image of a foppish Victorian in your head. The Hebrides is one of the 19th century's most original rethinkings of the orchestra, with a limpidity that prefigures early 20th-century impressionism; the Violin Concerto unfurls its miraculous, endless melody in a way that no other concerto approaches.
So much for the pieces we know. The scandal is that there are still hundreds of pieces that haven't been published, many of them jewels from Mendelssohn's uniquely productive youth - as well as masterpieces from his maturity that we never hear in concert programmes: the Psalm settings, which take Bach on at his own game; the bulk of the chamber music; and even the symphonies, apart from the Scottish and Italian. And the string quartets: if you want one piece to change your mind forever about Mendelssohn, to strip away misguided and racist critiques of his music, don't make the mistake I did and wait for the Emersons to play it in your local concert hall - listen to the Sixth String Quartet, now ·