For part-time chaplain Paul Song, Brixton Prison's oak-beamed 19th Century chapel was an oasis amid the hectic clamour of penal life.
But that was before Islamic extremists hijacked his Bible classes. One afternoon three inmates appeared in the chapel, interrupting a discussion on divine grace to loudly acclaim the killers of Lee Rigby, whose murder by jihadis on a South London street shocked the nation.
To the disbelief of Mr Song and his fellow Christians, the interlopers insisted that hacking to death the 25-year-old soldier was justified since, in their eyes, it avenged the killing of Muslims by British troops.
When Mr Song calmly tried to argue back, he was shouted down. Nor was it the only time his classes were to be similarly disrupted.