Religious conversion is an unusual subject for opera, whose concerns tend to be secular, even frivolous. It is arguably better suited to oratorio, a more static and sober form. When in 1838 Donizetti proposed to depict on the Neapolitan opera stage the martyrdoms of an early Christian and his wife, the royal authorities would have none of it. While this has been viewed as religious censorship, the King of Naples might sensibly have based his judgement on matters of taste and commerce. Who would go, the subplot love story notwithstanding? A performance of the French version of the opera Les Martyrs, albeit as oratorio, will be staged in London this week. The original Italian version of the opera was called Poliuto, after the hero and martyr Polyeucte. He was a Roman soldier who  joine