Here Gardiner betrays his origins as a child who grew up to be not so much God-fearing as Bach-fearing. Born in 1943, his fate was influenced by the war in a unique way, for the old Dorset mill where he grew up was also the place of safety for the most important and widely known image of Bach: the 1748 Haussmann portrait. This picture had been brought to England in 1936 by a German-Jewish refugee, Walter Jenke, who thereby saved it from the destruction that later befell his native Silesia. Gardiner's family looked after it and, as he recalls,   "every night on my way to bed I tried to avoid its forbidding stare". John Eliot grew up surrounded by Bach musicians, including Imogen Holst and Nadia Boulanger, who not only drummed harmony and counterpoint into him but bequeathed him her priceless collection of scores. Appalled by the mannered and saccharine singing of Bach he encountered when he went up to King's College, Cambridge, he resolved to incite a musical revolution in performance. The rest is part of the history of authenticity in music, including the lifelong quest to "demystify Bach".

It is hardly surprising that such a distinguished conductor takes it for granted that others will share his reverence for Bach the man, whose fiery personality he discerns both in his music and in anecdotes passed down by musicians — ripping off his wig and stamping on it in moments of rage, for instance. Gardiner describes his own reaction to seeing the Haussmann portrait again in Princeton some 60 years after it left his childhood home: "The overall impression is of someone a lot more complex, nuanced and, above all, human than the formal posture of a public figure would seem to allow." 

Yet Bach's humanity is inseparable from his faith in God's mercy. Blind, crippled by a stroke and dying, he dictated his "deathbed" chorale BWV 668a, Wenn wir in höchsten Nöten sein ("When we are in desperate straits"), which directly addresses God: "Turn not Thy gracious countenance / From me, a poor sinner." Nothing, it is safe to say, could be less congenial to the "Olympian" mentality of modern man. "It is Bach," Gardiner defiantly declares, "making music in the Castle of Heaven, who gives us the voice of God — in human form." For that reason Bach must remain a closed book to those for whom the category of divinity is meaningless, and hence deny that it is possible "to make divine things human and human things divine". Music — even Bach's music — cannot be "divine" unless God is a presence, unseen and perhaps unconscious, in our lives. We instinctively reach for theological metaphors when we experience the numinous quality of sacred art and music. But for these words to mean anything, we must have at least some confidence that the universe itself has meaning. Bach puts us back in touch with that numinous, on occasion even visceral, presence of the divine. And this involuntary response tells us that there is something transcendental within us, at the very core of our being, that recognises itself in this music. We are made in the image of God, the Bible tells us; in the same way, our music is a distant echo of Paradise. 

Bach's achievement is so colossal, so immortal, that it can obscure the fact of mortality, the finitude of humanity, which music exists to make bearable. We who doubt, as Bach himself doubted, the promise of eternal life can take comfort from music that gives us a foretaste of God's love. At the end of Dante's Divine Comedy (in Clive James's magnificent new translation), the poet's journey culminates in an ecstatic vision of  "the love that moves the sun and stars above". Music, like love, has the power to move the immovable, to melt the hardest heart, to bring hope to the hopeless. A musical legacy that encompasses all human life but also transcends it was bequeathed to us by Bach. Under Gardiner's expert guidance, the gates are thrown open to Bach's castle in heaven — a place that, like the isle in Shakespeare's Tempest, "is full of noises, sounds, and sweet airs, that give delight and hurt not". By hearkening to a music that is not quite of this world, we are granted an intimation of the next.