"We do not listen," Martin Luther was known to have complained, "even when the whole world and all creatures cry out to us and God is addressing us with his promises." For Luther, the Bible was not merely text, a book to be read silently, but living sound, or more precisely, "voice" – to be heard and listened to, and a ringing endorsement of music's role to express, adorn or enhance the word. JS Bach, who devoted much of his working life to setting music to biblical texts, forged a unique synthesis – between his music and the word of God.

For us today it is easy to be put off by the lurid imagery we find in Bach's anonymous cantata librettos. The experience can indeed be disquieting, yet surely no more so than with the equivalent in art? If we can accept without qualms the horrors on display in the paintings of Hieronymus Bosch or the gruesome etchings of Goya and not recoil, why should we have problems accepting Bach's way of clothing these unappealing words in music? For him, the two were indissolubly linked: he is in deadly earnest in probing the human condition, even at its murkiest, and every bit as intent as Goya was later on to rationalise the meaning of life. Get beyond the literal meaning of the individual words he is setting and it really is the music that counts. You might need to brace yourself against the periodic harangues, for Bach will never shirk an opportunity to box his listeners' ears in the interests of bearing witness to the truth – of what he sees to be tawdry or reprehensible in human behaviour.

Sir John Eliot Gardiner, photographed in March 2013. Photograph: Katherine Rose for the Guardian

This thought came to me at the start of our – that is, the Monteverdi choir's – Bach cantata pilgrimage back in Christmas 1999, when I first visited Buchenwald, less than five kilometres from Weimar where we were due to perform Bach's Christmas Oratorio. George Steiner describes Buchenwald – there up in the hills overlooking this most cultured and civilised city – as "hell made immanent". With that visit imprinted on our minds, we could sympathise with Bach in his minatory depiction of human nature as flawed. Though these are not his own words, mankind is "filth, stench, ashes and clods of earth", just as the 20th-century concentration and death camps were "the transference of hell from below the earth to its surface". Yet as we gathered to launch our pilgrimage, we recognised that Bach's music is one of the touchstones of civilisation – to be treasured every time we hear or recreate it. Its refusal to be silenced is overwhelming testimony to the strength and resilience of the human spirit. Our search for meaning, for answers to these intractable puzzles and contradictions, drew us together in the act of performing Bach's music on that occasion and all through that year, and has strengthened since then every time we get together to perform his music - just as we will be doing on Monday.

Bach can be witty, facetious, sardonic or just plain glum – it depends on his mood at the time. But give him trumpets and drums, and, as we all know from the Mass in B minor, there is no other composer before or since who can match the sheer thrill and zest of his festive music: the way it makes your spirit soar – vigorously martial one moment, transcendent the next, giving us an inkling or glimpse of how the angels might sound if only we could hear them play. The same is also true of his motets and cantatas. No other composer risked so much, encouraging his chosen performers – singers, trumpeters, oboists, violinists – to have their fling, their moments of virtuosic elation in the midst of rational order. You might heave a sigh of regret when the final cadence brings you back down to earth, but at the same time you know you have just been on a wild excursion with an amazing master of the musical stratosphere. In moments like these – and there are dozens of them scattered across the works we have chosen to perform on Monday – you know what Nietzsche meant when he wrote "music is something for the sake of which it is worthwhile to live on earth".

Bach's best vocal writing is all about helping his listeners to understand what choices they have: showing them heaven and then focusing on the real world and the available ways of dealing with it in terms of attitude and conduct. And when it comes to consolation, no one knew better than him how to deal with personal loss or to cushion the assaults of grief. By drawing together in the closing chorale all the layers of exegetical gloss he has given to the devotional themes in the course of a cantata, Bach is able to bring things back to the here and now of his listeners' concerns – to a sane present. The audience at the Royal Albert Hall will be invited to join in the singing of the Easter Chorale (BWV 4). His use of the chorale as a summation and as a thread running all the way through an individual cantata is one of his most brilliant and productive devices. It can be traced back to his childhood roots when as a treble he first sang Luther's hymns in the same church in Eisenach and probably on the very same spot where Luther himself had once stood and sung as a boy. Luther had quietly commandeered the folksongs of his day, transforming them into congregational hymns, turning what was a basic and easily approachable musical form into an enduring doctrinal tool, never more so than in his Easter hymn, "Christ lag in Todesbanden", which Bach clothed in music of unforgettable dramatic force in his cantata of that title.

In the course of his cantatas, Bach explored hundreds of different ways to highlight the chorale tune – with or even without words – like the gold or polychrome colouring in an illuminated manuscript that medieval scribes gave to important words.

• John Eliot Gardiner is conducting a "Bach marathon" at the Royal Albert Hall, London SW7, on Monday. It will be broadcast live on Radio 3.