Some biblical scholars have concluded from this that Bach acted like an astute textual critic, poring over Calov’s volumes and painstakingly comparing them, line by line, with other Lutheran Bibles. But there’s a simpler and more likely scenario, fully grounded in conservative 18th-century social and religious practices.

Picture the people of Bach’s household on free evenings, gathered in their living room for the activity of reading aloud. The children take turns reciting from a family Bible for practice in reading and elocution, not to mention spiritual edification. The patriarch follows along in his magnificent Study Bible, in part to make sure there’s no passage-skipping from the lectors, and in part to allow him to reach for his inkwell whenever he spots, compared with what he’s just heard, an error in Calov’s scriptural verses.

Tellingly, in something akin to what linguists call a mondegreen, Bach at several passages apparently misconstrued what the children — in this reconstruction of the scene — had said, and emended a scriptural verse’s legitimate Lutheran rendering to a similar-sounding but unattested wording. At Isaiah 16:8, Luther’s text reads: “its vine-branches are scattered, and over the sea.” Bach caught sight of Calov’s obvious typographical error “Fesser,” but he evidently misheard a lector’s utterance of the correct wording, and thus emended Luther’s intended “Feser” (vine-branches) to the biblically unattested “Fäßer” (wine-casks).

The Calov volumes also provide insight into Bach’s professional and personal concerns, showing that he understood himself less as a modern artist than as a preacher who was following his religious vocation. An annotation in Latin that the Crocker Laboratory physicists have filed under “definite Bach entries” makes for especially poignant reading, as it takes note of manifold passages in the Bible’s Solomonic literature speaking of how to find godly solace in a world that is hostile to people faithfully pursuing their divine callings. Sundry administrative records indicate that Bach often fell into trouble over philosophical differences with his employers about the place of music in worship and in education.

Only a handful of Bach’s entries in Calov concern music, and these have received the most extensive — indeed, typically the only — attention from biographers. Leading writers have striven to explain these marginalia as progressive. In truth, all of them straightforwardly reflect conservative Lutheran thinking. What they share as well is a premodern interpretive approach called “typology,” whereby events and principles in the era of ancient Israel act as “types” or “shadows” for their correlated “antitypes” or “substances” in the era of Christianity. Typology was looked upon less as a scholarly path to intellectual understanding than as a doctrinal path to spiritual comfort.