Yet the old-fashioned Lutheran view of Bach as the "Fifth Evangelist" is not completely at odds with Gardiner's portrait, as the celestial castle of its title indicates. As Gardiner relates, to mark the 250th anniversary of Bach's death in 2000, he conducted all 198 surviving church cantatas in one year, according to the liturgical calendar, in 50 cities and 13 countries. This Bach cantata pilgrimage — the recordings of which remain as an aural monument, both to the composer and to Gardiner — was unique, not only in musical but also in ecclesiastical history. The cantatas are of course only one of many facets of Bach's oeuvre; but their sheer quantity and quality, encompassing his entire career, provide an inexhaustible fount of inspiration. Christianity is central to Bach's music, not just because his was a deeply religious time and place, but because only a composer who saw music-making literally as worship could have produced works of such a kind and on such a scale. Bach annotated his copy of the Calov Bible, now preserved in Leipzig, with 348 marginalia, including the following, which might serve as his credo: "NB. Wherever there is devotional music, God with his grace is always present." Gardiner comments: "This strikes me as a tenet that many of us as musicians automatically hold and aspire to whenever we play music, regardless of whatever ‘God' we happen to believe in."

Bach's God, however benign, does not believe in letting humanity take it easy. Unlike his older contemporary Gottfried Wilhelm Leibniz, Bach never believed that his was the best of all possible worlds: on the contrary, its suffering was made tolerable only by redemption at the hands of Jesus, "the man of sorrows". Aged 22, he was already composing the miraculous work of consolation, the Actus Tragicus. Against George Steiner's dictum that Christian drama by definition cannot be tragic, Gardiner contends that the two Bach Passions, especially the later St Matthew Passion, belong squarely in the grand tradition of classical tragedy that extends from the Greeks to Shakespeare, Racine and beyond. He sees the revival of non-operatic music drama as "one of Bach's great achievements", pre-empting those of Mozart and Wagner: "Bach set in motion a new burgeoning of the genre, leading his listeners to confront their mortality and compelling them to witness things from which they would normally avert their eyes." However, Bach expected his audiences to use their imagination to visualise the tragic events evoked by his music, and Gardiner has an aversion to the staging of the Passions as "proxy-operas". They were written for the church, not the theatre; "extraneous aesthetic baggage" can only distract from and diminish this music. "Their power lies in what they leave unspoken," he concludes. "We ignore that at our peril." Amen to that.

One of the most absorbing aspects of Gardiner's book is his ambitious attempt to describe Bach's actual process of composition, as far as this can be gleaned from the evidence of manuscript scores and other contemporary sources. With the help of the self-effacing but omniscient Bach archivist Peter Wollny, Gardiner reconstructs the three compositional stages: inventio, elaboratio and executio. While composing a cantata (BWV 135) straight into full score — a rare gift, but also a necessity, for reasons of speed and economy — he ran out of space and had to wait five minutes for the ink to dry (no blotting paper yet) — "time perhaps for a coffee with Anna Magdalena, but perhaps also long enough to risk the train of thought (and invention) being broken". So Bach scribbled a little mnemonic at the foot of the page, a shorthand way of reminding himself how he meant the piece to continue. For Bach, invention was not the same as creation — only God could create ex nihilo — but was rather "an uncovering of possibilities already there". Asked for his secret, the old cantor is reported by his first biographer, Johann Nikolaus Forkel (who was writing within living memory), to have replied that it was just bloody hard work: "I was obliged to be industrious; whoever is equally industrious will succeed equally well."