Comment by TrapLord_Rhodo

From the Christian, anyone who wasn't part of the Christian or Jewish faiths was considered pagan. In the first century CE, Epicureans were part of the broader category of Hellenistic pagan philosophers—which included Stoics, Platonists, and others—who were polytheistic or at least non-Christian. So since Philodemus makes up most of the library here, it's pretty safe to call it a "pagan" library.