Nerva, the future emperor of Rome, called the Christian faith "Desiderata's lost cause". At the time the 'Universal Church' counted only a few thousand faithful rather than untold millions, and he didn't think it would last, as the Second Coming of the Messiah was looking more and more like a no-show. His blind friend Desiderata would demur and argue.
Then in AD 76 the first elected Pope was brutally murdered, and Desi realized that the very survival of her faith was at stake. Delegates from all over the empire had come to Rome for the Pope's ordination, but now suspicion reigned. How could they choose the victim's successor, while they could be electing his murderer? To restore confidence the killer had to be unmasked urgently.
However, solving a murder isn't simple when killing a man is not even a crime according to the law. In the end the 'lost cause' was not the one Desi expected.