That advice must have been a bitter pill for his three composer sons, who were indeed equally industrious but rather less successful. One of them, C.P.E. Bach, known as Emanuel, left us valuable insights into his father's work habits. Sebastian was a demanding teacher, but took great pains not to miss a spark of talent: "As for the invention of ideas, my late father demanded this ability [of his pupils] from the very beginning, and whoever had none he advised to stay away from composition altogether." Emanuel evidently passed this test, and relished seeing his father's reaction on hearing a work for the first time: "When he listened to a rich and many-voiced fugue, he could soon say, after the first entries of the subjects, what contrapuntal devices it would be possible to apply, and which of them the composer by rights ought to apply, and on such occasions when I was standing next to him and he had voiced his surmises to me, he would joyfully nudge me when his expectations were fulfilled." On this uncanny mastery of elaboratio, Gardiner comments: "Like a chess grandmaster, Bach is able to predict all the next conceivable moves."

Impressive as was Bach's sheer musicianship, it does not bring us to the heart of the matter. Why does this composer, above all others, inspire a down-to-earth Englishman such as Gardiner to break into ecstatic prose about "his balletic joy in the praise of his maker and his total certitude in the contemplation of death"? Nor is there anything new about this. In 1827, Gardiner's early 19th-century precursor in the rediscovery of Bach, the Berlin choirmaster Carl Friedrich Zelter, wrote to his friend Goethe, then the greatest living poet: "Could I let you hear some happy day one of Sebastian Bach's motets, you would feel yourself at the centre of the world, as a man like you ought to be. I hear the works for the many hundredth time, and am not finished with them yet, and never will be." Gardiner does not merely see Bach as a source of spiritual solace or sentimental uplift, but as a truly Christian antidote to the specifically religious vices of intolerance and hypocrisy — the "redemptive way back" to what Bach himself called "good neighbourliness".

Gardiner grapples with the mystery of Bach's genius most valiantly in his penultimate chapter on the B minor Mass, whose "Byzantine or Venetian splendour" he sees as a final, monumental demonstration of the composer's "habit of perfection", by setting the most universal, "catholic" text of all: the Ordinary of the Mass. This, Gardiner claims, is Bach's Nunc Dimittis, the consummation devoutly to be wished and his farewell to the world. Recent scholarship, notably Christoph Wolff's, suggests that although Bach's "Great Mass" has long been known to be a palimpsest of "parodies" or borrowings from earlier works, assembled over decades, much of the music breaks new ground. Indeed, the Et Incarnatus of the Symbolum Nicenum may be the last completed movement Bach ever wrote. This sublime invocation of incarnation and redemption may owe something to Pergolesi, but is unprecedented in its synthesis of the joy and grief of lost innocence. In the subsequent return to doctrinal affirmation, a "shadow passes over this illuminated missal" in the passage that evokes the forgiveness of sins and the resurrection of the dead. Bach's usually rare corrections are scrawled all over this section of the autograph score. This, the "eschatological crossroads of the entire Mass", is for Gardiner perhaps the darkest moment in Bach's entire career, when "we are privy to his vulnerability and his doubts", before the crisis is resolved, the anxious et expecto ("and I look for") becomes a confident credo resurrectionem mortuorum in a "jubilant collective sprint to the finishing line". Gardiner is emphatic that the B minor Mass has the power to inspire even the most godless, precisely because it "does not rely on credal orthodoxy, odd though that might appear". Rather, Bach's "art celebrates the fundamental sanctity of life". So it does, but "sanctity of life" is the last thing that the new atheists wish to celebrate. Bach's sacred music is accessible to all, but as Gardiner's analysis of the B minor Mass shows, to ignore his struggles to make sense of "credal orthodoxy" and just let the music wash over you is to render it two-dimensional.