Martyrdom is dangerously contemporary. We do not live in a post-religious age: despite Nietzsche’s best efforts, God is not dead. Whether He is alive, beyond existence or merely too busy with the weather these days, there is no doubt that our world remains tragically full of people who will cheerfully die for – and kill for – their own vivid idea of God. This was no less true for Handel. In 1750, Handel’s Theodora, an oratorio which calls for pity, freedom and religious tolerance, was an instant flop. It was partly because, five years after the Jacobite Rebellion, Theodora was out of sync with the jubilant Protestant majority; partly because, as a Christian story, it didn’t appeal to Handel’s otherwise keen Jewish fans; and partly, as Handel himself wryly remarked, because Theodora is a story of virtue, “the Ladies will not come.”

Theodora is a Christian story, though not a Biblical one: and it is undoubtedly a tale of virtue. Theodora defies a Roman edict to sacrifice to Jove, and is initially condemned to a brothel as punishment (her chaste response: “Oh, worse than death indeed! Lead me... to the rack, or to the flames, I’ll thank your gracious mercy”). Happily, Didymus, a Roman converted to Christianity for love of Theodora, saves her, and is condemned to death in turn. Once the death sentence is extended to Theodora, she joyfully comes forward to be martyred in Didymus’ place. As both vie for death to save the other, the harassed Roman governor Valens decrees a double execution, and the bewildered Romans marvel “How strange their ends, And yet how glorious”. The climax of Theodora shows the impossibility, for those for whom death is a threat, of political control over those for whom it is a privilege.

Although Theodora is full of brilliantly drawn characters, each with their own unique music, it is the character of Didymus – a Roman, and a Christian, sung with liquid clarity here by Tim Mead – who is the hinge, epicentre and real star of this thoughtful oratorio. Mead’s supple countertenor floats effortlessly and dexterously through Handel’s strong, clear lines and intricate passages of coloratura, as Didymus chooses between his friends and his beloved, his comrades and his faith, resisting aggression and pleading for mercy with equal fervour. He alone can move between two camps, accepted by each; he alone understands the motivation of both sides, yet chooses freely for himself. And his music (Handel’s last castrato role) is utterly sublime. So often, the English convention of repeating one phrase endlessly can feel repetitive; but Didymus’ arias feel more like soliloquies, his mind examining his actions before returning with increasing certainty to his chosen course. I have never enjoyed Handel more than hearing Mead sing “The raptur’d soul”, “Kind heav’n”, “Deeds of kindness” or “Streams of pleasure”, a Christian vision of heaven – eerily reminiscent of the much-touted rewards of Islamic martyrdom, a parallel which adds a dark twist for a modern audience in the age of the suicide bomber.

Sarah Connolly is fabulous throughout as Theodora’s friend Irene, whose strong faith is made human by her tremulous doubts and vibrant love of life. Amidst a great overall performance, Connolly’s passionate plea “Defend her, heav’n” is supremely well-judged and sung with real emotion. Rosemary Joshua is an appealing, convincingly idealistic Theodora, whose voice seems perfectly suited to “Angels, ever bright and fair”; her duets with Didymus are also beautiful. Neal Davies delivers Valens’ distinctive music with bloodthirsty gusto and wonderful resonance in an assured, inspiring performance which simply shivers with energy. Kurt Streit, who stepped in at short notice as Septimius, does valiant justice to very demanding music, as well as communicating a realistically unheroic character whose genuine desire for compassion is outweighed by his fear of authority: “We can only pity / Whom we dare not spare.”

The choir of Trinity Wall Street develop a rich, warm tone throughout the piece. “He saw the lovely youth”, a composition which Handel personally prized over his Hallelujah Chorus, is a magnificent choral work, moving from deathly harmonies to rejoicing humility, celebration and awe: its shape, content and structure are all extraordinarily beautiful. For me, “Blest be the hand” was also an absolute triumph. Harry Bicket, conducting from the keyboard, produced a glowing period sound from The English Concert, who played with stately drama and intensity.

Profound, fair and thought-provoking, Theodora argues for faith as a personal journey, not a political decree. As Didymus sings: “Ought we not to leave / The free-born mind of man still ever free? / Since vain is the attempt to force belief / With the severest instrument of death?” It is not only ancient pagans who try to win over hearts and minds with mortal threat: Handel’s Theodora is an unflinching meditation on the darker moments in the evolution of Christianity itself, not to mention certain political entities in more modern conflicts. Ideological death-dealers of all creeds, take note: martyrdom is a very private kind of victory.