I love the amorous mayhem of Handel’s operas, but have always had my doubts about his oratorios, especially the Messiah. First there’s the bossy compulsion to stand during the “Hallelujah” chorus, just because a spurious tradition says that King George II did so in 1743; once hoisted upright, you have to fidget through endless awkwardly mis-accentuated iterations of “for ever and ever”. I’m also puzzled by the quirks of the biblical text, which are underlined by musical repetition. The soprano rhapsodises like a fetishist about the “beautiful feet” of those who preach the gospel, and the tenor prophesies that “every valley shall be exalted” by the saviour: will salvation really reflate those sagging hollows in the landscape?

But after reading Charles King’s Every Valley: The Story of Handel’s Messiah, I have been converted. King doesn’t exactly explicate the phrase he takes for his title, but he points out that Martin Luther King often quoted it in his orations about civil rights, so I probably shouldn’t quibble. More importantly, his book humanises the work’s exalted creators and demonstrates that the Messiah is not a pompous manifesto of faith but a troubled, often desperate quest for consolation. Despite those hectoring hallelujahs, what moves King is the oratorio’s prescription for overcoming personal misery.

King begins with the librettist Charles Jennens, a rich but miserably hypochondriac book collector, haunted by the suicide of a brother who had cut his throat and then for good measure defenestrated himself at the Middle Temple. The biblical quotations that Jennens collaged together for Handel had a secret psychological plot: they were “an affirmation of something Jennens himself had always found it hard to believe in”. Handel too, as seen by King, is a worldly figure, an obese epicure “with prominent jowls and a chin that stairstepped down into his cravat”. His statue in Westminster Abbey shows him pointing a finger heavenwards as he displays a page of the Messiah score that declares his faith in a redeemer; a trumpet pokes out at us, ready to amplify the pious message. But far from ingratiating himself with God, Handel had a diabolical temper and once silenced the uncooperative soprano Francesca Cuzzoni by telling her “I am Beelzebub, the chief of the devils!” and threatening to toss her out of the window.

The soprano who appeared in the first performance of the Messiah, Susannah Cibber, was less turbulent in rehearsals than Cuzzoni but even more flagrant offstage. She was trafficked by her husband, who sold her sexual favours to a crony to defray his debts. Sneaky servants drilled holes in walls to watch her lover “put his privy member between her legs”, and scandal sheets reported on her sensational abduction from a country retreat by a posse of armed thugs. After these infamies, her performance in the Messiah was an attempt to relaunch her career and launder her sullied reputation: early audiences took her delivery of Handel’s solemn recitatives about Christ’s agony to be a personal appeal for restitution, since she, too, was “despised and rejected” and “acquainted with grief”. Supplied with this subtext, the oratorio becomes something of an enjoyably tawdry soap opera.

King, a professor of international affairs in Washington DC, does a fine job of implicating Handel in the conflicts and contradictions of an unsettled society. He first travelled from Hanover to London in 1710, arriving in what seemed to be “a failed state, mired in revolution, political conspiracy and murder”. Installed at the English court, he unofficially spied for his royal patrons at home in Germany while composing music to glorify the local Hanoverians, who captured the crown from the Stuart dynasty in 1714: King argues that the chorus in the Messiah that salutes Christ as “Wonderful, Counsellor, the Prince of Peace” was a coded tribute to George II. Handel’s colleague Thomas Arne, who happened to be Susannah Cibber’s brother, supplied the monarchy with its imperial anthem in “Rule, Britannia!”. Although Britons here boast that they “never never never shall be slaves”, they happily profited from the enslavement of others. Both Jennens and Handel were clients of the South Sea Company, whose “signature money-making venture”, as King notes, was “the involuntary transport of human beings” from Africa to the American colonies. Music, the airiest and most spiritual of arts, is murkily embedded in the realities of politics, commerce and inhuman exploitation.

All the same, King derives an irreligious solace from the Messiah. He turned to it, he reveals, after a time of troubles – first, the morbidly anxious pandemic, then his wife’s serious illness, finally the ghastly day in 2020 when, blocks from his house, Trump’s crazed vigilantes overran the US Capitol. The “messed-up state of everything” left King with a desire for “healing light”; this was supplied by Handel’s tenor who, before he exalts the valleys, conveys God’s reassurance by declaiming, “Comfort ye, my people”. Listening to this at home on their antiquated gramophone, King and his wife burst into grateful tears.

After the recent election, they will be in need of further comfort. But the Messiah, King says, is an admonition “to live bravely in the face of disaster and defeat”, and I hope it sustains him during the next four years of what the tenor rightly calls “iniquity”.

• Every Valley: The Story of Handel’s Messiah by Charles King is published by Bodley Head (£25). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply