When her pack is stolen, a spirited Corinthian girl must reclaim it from thieves and the important letter within which she promised to deliver to Rome. (Possible script title: ‘Phoebe’)

PHOEBE

When the backpack of a Corinthian girl entrusted to deliver a letter imperative to solving the riddle of?the debilitating supernatural disease?plaguing Rome is stolen by a group of?goblins, she?must find a way to reclaim it? before they learn it's secrets and destroy the Empire, it's citizens, and all of humanity.

Agreed with DPG. Worth noting though that while the Emporer is not the antagonist, he is the stakes at hand - naming him clarifies this. However, the plot of stealing the epistle (the A plot of the story) seems to pale in comparison now to the potential plot of helping hundreds of people in Rome. Perhaps best to describe the actual challenge the girl will face in getting the epistle back, right now it's only lightly brushed over.

E.g:
After the Apostle Paul's epistle is stolen from her travel pack by the republic's most notorious robber, a spirited Corinthian girl must get it back to inspire Christians persecuted by the? Emperor Nero.

I suggest naming the villain responsible for the persecution, Rome's greatest fiddler. So:

When the Apostle Paul's epistle to the Romans is stolen from her travel pack, a spirited Corinthian girl must get it back to inspire Christians being persecuted by the Emperor Nero.
(31 words)